Photolysis (a 200 W high pressure mercury lamp) under oxygen stream of ethylene dithioacetals of 5, 6-dimethoxy-α-indanone, α-and β-indanone, α-tetralone, 5α-and 5β-cholestan-3-ones, cholest-4-en-3-one, and cyclohexanone in an appropriate solvent (nhexane, ethanol or benzene) was carried out to give the parent ketones. Ethylene dithioacetal of cholest-5-en-3-one on photolysis followed by NaBH₄ reduction gave cholest-4-en-3β-o1, showing photo-induced isomerization of the double bond.