Vacuum pyrolysis of 5-silaspiro[4.4]nona-2,7-diene (5a), UV irradiation of matrix-isolated 1,1-diazido-1-silacyclopent-3-ene (6), and vacuum pyrolysis of 6 resulted in the formation of 1-silacyclopenta-2,4-diene (1a), characterized by its IR and UV-vis spectra. The 3,4-dimethyl analogue lb was generated by vacuum pyrolysis of 5b, the 2,3,7,8-tetramethyl derivative of 5a. Photoreversible interconversion of 1a with 1-silacyclopent-3-ene-1,1-diyl (2a), 1-silacyclopenta-1,3-diene (3a), and 1-silacyclopenta-1,4-diene (4a) and the analogous interconversion of 1b with 3,4-dimethyl-1-silacyclopent-3-ene-1,1-diyl (2b), 3,4-dimethyl-1-silacyclopenta-1,3-diene (3b), and 3,4-dimethyl-1-silacyclopenta-1,4-diene (4b) have been observed upon irradiation at selected wavelengths. Full vibrational assignment in the IR spectra of 14 has been proposed on the basis of restricted Hartree-Fock calculations for 1a-4a and of literature data on related molecules. The UV-vis spectra are interpreted with help from multireference configuration interaction calculations, but the difference between 3 and 4 is accounted for even at the Huckel level. The significantly lower Si=C stretching frequencies as well as the UV absorption bands of 3 and 4, red-shifted relative to 1-methyl-1-silene, provide strong experimental evidence for Si=C-C=C and C=Si-C=C pi-conjugation.