A process is disclosed for the dehydrohalogenation of partially halogenated ethanes of the formula C.sub.2 H.sub.a Cl.sub.b F.sub.c where a is an integer from 1 to 4, b is an integer from 0 to 3 and c is an integer form 1 to 5 to produce olefins selected from the group consisting of C.sub.2 H.sub.a-1 Cl.sub.b-1 F.sub.c and C.sub.2 H.sub.a-1 Cl.sub.b F.sub.c-1. The process involves contacting C.sub.2 H.sub.a Cl.sub.b F.sub.c with a zeolite selected from the group consisting of NaX and CsY. Selective reaction of one isomer from a mixture of two isomers is disclosed as a means for purification of the relatively unreactive isomer. Also disclosed is a process for producing perfluorocyclobutane which involves contacting CHF.sub.2 CClF.sub.2 with such zeolites.