Thermally generated dimethylsilylene reacts with 1,4-diphenyl-1,3-butadiene to give 1,1-dimethyl-4,5-diphenyl-1-silacyclo-2-pentene. A mechanism involving homolytic C–C bond cleavage of the intermediate vinylsilacyclopropane is suggested. Dimethylsilylene also undergoes a novel silylene–silylene exchange reaction with 1-methyl-1,2,3,4,5-pentaphenylsilole.