Direct photolyses of 1,1-dimethyl-1-silacyclobut-2-ene (1) at 214 nm in tert-butyl alcohol gave 29% tert-butoxydimethyl(2-propenyl)silane (2), 4.3% (Z)-tert-butoxydimethyl(1-propenyl)silane [(Z)-3], 6.9% (E)-tert-butoxydimethyl(1-propenyl)silane [(E)-3], and 5.9% tert-butoxytrimethylsilane (4) at 49% conversion. Products 2-4 were primary, and the total quantum yield of formation was 0.11. Use of tert-butyl alcohol-0-d led to >98% monodeuteration: 2-dl had 93.5% of the label at cl and 6.5% at c3 of the 2-propenyl group, (E)- and (Z)-3-d1 [(E),(Z)-3-dd were labeled >95% at C3 of the 1-propenyl group, and 4-d1 was deuterated at a silyl methyl. Cleavage of a silacyclopropylmethylene intermediate to 1,1-dimethyl-1-silaethene accounts for the labeling of 4-d1 and the deuteration of 2-d1 and (E),(Z)-3-d1 suggests the intermediacy of 1,1-dimethyl-1-sila-1,3-butadiene, while the 13% contribution by a photoprotodesilylation pathway explains the partial C3 labeling of 2. Additional evidence for silicon-carbon doubly bonded intermediates is the formation of 31% 2-methoxy-2,4,4-trimethyl-3-vinyl-2,4-disilapentane and 4.8% 2-methoxy-2,4,4-trimethyl- 2,4-disilapentane upon 214 nm photolysis of 1 with 0.25 M methoxytrimethylsilane in pentane. Product yields versus tert-butyl alcohol concentration were determined to probe whether equilibrating, ground-states-cis (gauche) and s-trans conformers of silabutadiene were involved in the formation of the 2 and (E),(Z)-3. Optimized geometries using the 6-31G(d) basis set were calculated for the cis, gauche, and s-trans structures of 1-sila-1,3-butadiene and 1,3-butadiene, as well as relative energies and barriers for interconversion of the gauche and trans species at the MP2/6-31G(d) level.