Persilyl-substituted silacyclopropenes, (R3Si)(2)SiC2(SiMe3)(2) (1a: R3Si = (Pr3Si)-Pr-i, 1b: R3Si = (Bu2MeSi)-Bu-t), were photochemically isomerized to 1-silaallene derivatives, (R3Si)(2)Si=C=C(SiMe3)(2) 2, at wavelengths longer than 340 nm. These compounds subsequently photoisomerized to acetylene derivatives, (R3Si)(2)(Me3Si)Si-C=C-SiMe3 3, as a final product. The existence of 1-silaallene 2 was demonstrated spectroscopically as well as by a trapping reaction with water.