A new S-derivative of riboflavin was prepared by the reaction of riboflavintetraacetate with P_2S_4 in benzene or pyridine solution. This compound, mp. 234〜235℃, C_<25>H_<23>O_3N_4S, reacts readily with 2NNaOH in methanol solution to form free thioriboflavin, mp. 232〜234℃, C_<17>H_<20>O_3N_4S.