Paterno-Buchi photocycloaddition of silyl O,S-ketene acetals (SKA) 1 and aromatic aldehydes 2 was investigated in detail. The photoreaction of aldehydes 2a-d with beta,beta-dimethyl-O,S-SKA 1a-e has been found to give, regio- (ca. 70/30 to 90/10) and stereoselectively (ca. 70/30 to 90/10), trans-3-siloxyoxetanes 3 independent upon the aldehyde, the substituents SR1 and SiR3, and reaction medium (solvent and salts). The triplet 2-oxatetramethylene 1,4-diradical T-1,4-DR is reasonably proposed as a common intermediate. The regioselectivity is rationalized by (1) the relative stability of 1,4-diradicals and (2) the relative nucleophilicity of sp(2)-carbons in O,S-SKA. The trans-selectivity is explained by the sulfur atom effects in O,S-SKA 1, which control the approach direction of the electrophilic oxygen of triplet n pi* aldehydes to the nucleophilic alkene. The fast ISC process of the triplet 1-alkylthio-1-siloxy-2-oxatetramethylene 1,4-diradical T-1,4-DR in competition with the bond rotation has been proposed. The substrate-dependent formation of 3fb from E-or Z-1f supports the hypothesis. The S-directed regio- and diastereoselectivity are found for the first time in the present study.