Four 5,8-epidioxy sterols were isolated from the marine sponge Plakortis simplex. Their structures were completely determined by an extensive NMR analysis and comparison with NMR data of similar compounds for absolute stereochemistry of the side chain. The compounds were assigned as 5,8-epidioxy- (24S)-ethylcholesta-6,22(E),25-trien-3-ol(1), 5,8-epidioxy-(24S)-methylcholesta- 6,22(E)-dien-3-ol(2), 5,8-epidioxycholesta-6,22(E)-dien-3-ol(3) and 5,8- epidioxycholesta-6-en-3-ol (4).