Thiobenzoic acid O-esters of part structure X·[graphic omitted]H·[graphic omitted]·O·C(S)Ph (X = olefinic linkage, aryl residue, or other conjugating function) are smoothly photolysed (in general in high yield) to the corresponding olefin and thiobenzoic acid; similar O-esters with the X function not in potential conjugation are photolysed only sluggishly.