The structure of strepsilin was examined by infrared spectra. The C=O stretching frequencies of strepsilin at 1745 cm^-1 (in dioxane) and its shift to 1770 cm^-1 by the methylation of hydroxyls indicated that strepsilin possesses a hydrogen-bonded carbonyl of phthalide ring, which accounts for the structural formula (Ia) of strepsilin. The structures of phenolic acid, m.p. 308°, and the yellow crystalline compound, m.p. 247°, which were derived from strepsilin, were discussed from their infrared spectra.