On treatment with [Ir(PMe3)(2)(acac)] at room temperature, 1,2,3-triphenyl-3-vinylcyclopropene undergoes ring opening accompanied by rearrangement to give, instead of the expected 1,2,3-triphenyliridacyclohexadiene complex, a crystallographically characterized 1,2,4-triphenyliridacyclohexadiene complex containing cis-phosphine ligands. Studies with H-2- and doubly C-13-labeled vinylcyclopropenes, the syntheses and characterization of which are also reported, show that this process involves a rearrangement of the carbon skeleton and not a substituent shift. The corresponding 1,2-diphenyl-3-vinylcyclopropene undergoes iridacyclohexadiene formation without any rearrangement. On heating at 90 degrees C, each iridacycle converts to its corresponding isomer containing trans-phosphine ligands without any skeletal or substituent rearrangement of the metallacycle, as evidenced by absence of change in the labeling pattern. At higher temperatures, further rearrangement occurs in the case of each metallacycle, which does not alter the metallacyclohexadiene backbone, but rather exchanges the substituents of the alpha and alpha' carbon atoms. This rearrangement is shown to occur even when there is no driving force due to relief of steric effects. Mechanisms for each rearrangement are proposed and discussed.