Irradiation of isothiochroman-4-one in cyclohexane (high pressure mercury lamp, Pyrex 7740 filter) produced thiochroman-3-one (21%). Under similar conditions 7-methoxy-, 8-methyl-, 3-methyl-, and 3,3-dimethyl-isothiochroman-4-one rearranged to 6-methoxy- (40%), 5-methy1- (30%), 2-methy1- (30%), and 2,2-dimethyl-thiochroman-3-one (37%), respectively. 1,2,3,4-Tetrahydro-1-keto-3-thiaphenanthrene failed to undergo the rearrangement reaction. Irradiation of 5-methyl-2,3-dihydro-2H,6H-thiopyran-3-one under similar conditions resulted in rearrangement to 2-(2-prop-1-enyl)-1-thietan-3-one (28-35%). The pathway for the photochemical conversion of isothiochroman-4-ones into thiochroman-3-ones is discussed. © 1969, American Chemical Society. All rights reserved.