There have been two recent reports on the photodirerization of substituted cyclopropenes. A. R. Stechl has found' that 1,3,3-trimethylcyclopropene, on irradiation in the presence of benzophenone , produces two isoneric dimers, both tricyclo_~,1,0,02~47-hexane derivatives. Obata and Moritani' report that 1,2-diphenylcyclopropenyl methyl ketone also dimerizes to a tricyclohexane on irradiation. We have been examining the photochemistry of some cyclopropenes, and wish to report on a subtle but interesting facet of the dimerization of 1,2,3-triphenylcyclopropene. When a degassed 0.04 M solution of 1,2,3-triphenylcyclopropene (I) in benzene is irradiated with 3130 1 light there are no reactions of quantum yield greater than 10w2. If a sensitizer such as benzophenone is added to the solution and filtered light of 3660 i used to insure that only the sensitizer is excited, a high quantum yield reaction occurs giving the two dimeric products shown below (II and III). II 6a III 4046 It is clear that the dimerization occurs via the triplet state of triphenylcyclopropene. We find there is no change in the ratio of the two products with sensitizers of different triplet energy and different *NIB Postdoctoral Fellow, 1965-66.