Some qualitative observations on the catalytic activity of Lewis acids in converting ergosteryl acetate into its 5α,8α peroxide are presented. The peroxide has an inhibiting effect on the reaction, which is ascribed to complex formation between it and the Lewis acid catalyst. This complex formation is used to explain the dual thermal and photochemical activity displayed by FeCl3 and MoCl5. A mixed oxygenation experiment with ergosteryl and lumisteryl acetates indicates that singlet oxygen is not an intermediate in this reaction. ��� A model for the reaction, involving formation of a diene cation radical within a charge-transfer complex, is advanced.