It is a mixture of propane and butane.
The EPA says it is an unacceptable substitute:
http://www.epa.gov/ozone/snap/refrig.../unaccept.html
There are EPA approved substitutes that wont turn your car into a plasma cutter if you get a high side leak and it gets lit off:
http://www.epa.gov/ozone/snap/refrig....html#mvacpubs
IMHO: This is a case where I agree completely with the EPA's stance. Stick with the original refrigerant if you can, and convert to an approved substitute when the original is no longer available in your area.
You cars fuel system was designed to carry a flammable, your A/C was not.