OVERVIEW: WHY THE UK?
Bizarrely, considering the language of their packaging and their nationality, Mexican dolls seem to be almost native to the United Kingdom. Some have suggested that these Mexican dolls were produced for the Spanish line and that the Mexican dolls in the UK have simply travelled across Europe. I don't endorse this idea for one moment. Firstly, Spain had their own line of dolls manufactured on home soil - the Spanish dolls have a seperate section but it is interesting to note that the four Spanish dolls I have were all purchased from Spanish soil originally and yet I have never seen a Mexican doll on Spanish ebay or from a Spanish seller. Besides, Spanish and Mexican Spanish is not identical...so it seems likely that the boxes were not perhaps even relevant to Spain.
So with that theory neatly pushed to one side, the question still remains. Why Mexican dolls in the UK?
My belief is based somewhat on my experience with Hasbro in my other big collection - My Little Pony. In several countries in Europe, presumably because of difficulties with or maybe cost of licensing certain ponies from Hasbro Inc, the local Hasbros licensed and issued their own versions of these characters - made in Italy, Spain, France and several other countries besides. It is my belief that the Mexican dolls were the UK equivalent from the Jem line. It seems more than likely to me that the Mexican dolls were a cheaper option for the UK branch of Hasbro to import than perhaps from Rhode Island's Hasbro branch. Equally, with the UK being a year behind the USA, it might well have been the only means to continue the sale of apparently "first edition" dolls after the line had changed in the United States.
I do not know if Mexican dolls were sold anywhere else in the world, but from the quantity located on British shores compared with anywhere else (every single one of my Mexican dolls has come from the UK) the idea that all of them are accidental imports seems a little bit airy fairy to me. The only solution is that, wherever and whyever they were produced, Mexican dolls were an official part of the UK's Jem line.

My own Mexican collection is ALMOST complete but there are still gaps and I'd appreciate any help from anyone to fill those gaps. You can find out what I still need by clicking this link:)

HOW DO I TELL IF MY DOLL IS A MEXICAN DOLL?
Well...it's fairly simple, actually. If your doll has all the following features:

• pale skin
• very shiny body
• hair of a slightly different shade to a normal doll
• no stamp of manufacture on the head or the body
• proportions make her slightly taller than a normal Jem doll
• If she is Jem herself, she has large red starcaps.

HOWEVER....
There are some Mexican dolls which do NOT fit all of the above criteria. I know there are certainly some Mexican hybrid dolls, with normal second year heads, and the very shiny body...these can prove confusing, because they do have the hasbro stamp on the head, if not on the body. What the origin of these are is unclear, but they appear to have been sold in Mexican packaging, so I'll classify them here as 'hybrid Mexican dolls' These were possibly the end of the line dolls for the Mexican production, since they seem to have been dolls from the Second year anyway, and were the means of using up second year leftover parts in the same way as Superstar dolls used up bits from the First Year.

WHAT'S THE DIFFERENCE BETWEEN MEXICAN AND SPANISH JEM DOLLS?
Simple. Spanish dolls have 'Made In Spain' stamped on the back. :-D If you have them in package, and aren't sure whether your doll is Spanish or Mexican, you can tell by the text on the box. Mexican dolls have 'Jem: Es Fabulosa!' whilst Spanish dolls have 'Jem: Verdaderamente Fantastica!'. The text on the back of the box is also different.
If you're lucky enough to have the dolls boxed, also, you'll notice that the Mexican dolls have instruments which resemble second year instruments more than first year ones, even though they are dressed in first year attire. Spanish dolls have basically first year style instruments, even if there is a slight colour variation. Mexican dolls also have cassettes with songs IN MEXICAN SPANISH. Spanish dolls have cassettes IN ENGLISH.
For examples of Spanish dolls and accessories, see the SPANISH part of the page.

WHAT MEXICAN DOLLS EXIST?
There are to my knowledge seven characters made in Mexican form - Jem/Jerrica, Kimber, Aja, Shana, Pizzazz, Stormer and Roxy. As yet noone has discovered a Mexican Rio. Ironically those new characters from the second year, such as Mexican-American character Raya was NOT made in Mexican form (as far as we know!!) Equally there is no Mexican equivalent of Jetta, Clash, Synergy or any Starlight Girl.
Each of the seven characters that do appear in Mexican form can theoretically be HYBRID or PURE Mexican dolls.

SO TO CLARIFY, WHAT IS THE DIFFERENCE BETWEEN A HYBRID AND A PURE MEXICAN DOLL?
Pure Mexican dolls have no stamping on them at all (manufacturer's stamp). If you look at a normal Jem doll you will find that she is stamped Hasbro between the shoulderblades and also on the back of her head in some way or form. A pure Mexican doll has none of these markings and often her features are not identical to an ordinary issue doll. These changes in features include differing shades of hair and makeup and often a slightly dreamy look about them. To the left is an example of a pure Mexican body from my Kimber doll. As you can see, she has absolutely no markings on her back.

A Hybrid Mexican doll is as it sounds - a doll with a Hasbro stamped head but no stamp on her body. These are not put together jobs since collectors have acquired them as close to mint as is possible this long after their issue. Most of the dolls who are Hybrids have Second Edition heads - as you can see from the scan to the right of my Hybrid Aja, she even has her green earrings to prove it! However, as with the pure Mexican doll, she has nothing stamped between her shoulderblades. The back of her head (you can just see in the picture) is stamped Hasbro.
Jem vs Pranceatron also has a fabulous page discussing Hybrid Mexican Shana and there are more pictures and information snippets on the individual pages for the doll.

TYPICAL STANDS AND COMBS FOR MEXICAN DOLLS (EXAMPLES BELONG TO MEXICAN SHANA)
The Stands and combs of Mexican dolls are also identifiable as such despite being very similar to ordinary stands at first glance. The example I'm using is from my Mexican Shana, but this is also true of Misfit dolls who have neon yellow stands that follow the same rules.
The key thing with these is that there is no TM stamp after the word JEM. This would further support the idea that the Mexican dolls were licensed by Hasbro UK for production as a cheaper manufacturing alternative, but not by Hasbro INC itself. The date of their issue also matches the stand style - all of them are cloudy stands much like Second Edition doll stands. This fits exactly also with the style of instruments (See Roxy and Shana's pages) and the heads of Hybrids (See Roxy and particularly Aja's pages) that Mexican dolls exhibit.
The combs are in a similar vein. The comb shown belongs to Mexican Shana - believe it or not. Although it's a different shade from her First Edition and her Second edition comb, this is the one that came in her box :) It too lacks the TM after the JEM logo.

EXAMINING THE MEXICAN BOX (EXAMPLE BELONGS TO MEXICAN SHANA)

Mexican Shana's box is, I would imagine, much the same as the regular First Edition Shana box, although sadly at this time I have no regular First Edition Shana box to compare it with. However, here are a few pictures of the Mexican box anyway. Note that all text is in Mexican Spanish (There is a distinct difference in box text between Spanish and Mexican Spanish) The first picture shows the box as a whole. Sadly the 'Es Fabulosa' is hidden from view in this picture, however the artwork is much the same, if not *the* same as a regular First year box. The box also is seales in the same way, with tabs rather than the flaps of a Spanish box. Spanish boxes also say Jem  'Verdaderamente Fantastica!' instead of Jem 'Es Fabulosa'. The top of the box - clearly indicating the Spanish language origin of the package. Notice Shana's name has no 'TM' but instead 'MR'. And finally, if not least, the information regarding the cassette. On a spanish box, the titles of the songs can be found in either English or Spanish but the cassette appears always to be in English. Mexican dolls are unique for their foreign language cassette tapes. Clearly here you can see the titles of the songs included on Mexican Shana's tape.