3-dichlorobistrifluoromethyl- (I) and 1,3-dichlorobistrifluoromethyl-cyclopropene (II). The 3,3-dichloro-compound (I) is isomerised to (II) by heat, light, or chemical catalysis, but high yields of either pure cyclopropene may be obtained by modification of the reaction conditions. The cyclopropenium ion (III) is formed when either (I) or (II) is treated with antimonypentafluoride.