Mixed freons

I was told that my AC is charged with both R12 and R134A freons, and it should have been only R134A. The AC was not cooling well, and they topped it with the wrong freon type, solving the problem momentarlly. My 92 535i has 130,000 mi, and had compressor/drier/expansion valve replaced at 40,000 mi (live in Houston - 10 mo/year air is on). Now I am for a big problem, having used the mixed freon types for two months ?

No. Find an A/C specialty shop that will recover the contaminated freon you have, purge the system then evacuate and recharge it with R134a.

Just FYI uir in 92 they still did r12, in 93 it was greatly illiminated from the market but there were still a few limited cars that had them. Make sure you read up so you use the correct kind. Even though its most likely r134 just doubble check to be 100% sure.