Elsinochrome A (EA) is a natural perylenequinone known for high photosensitization activity. There are a limited number of studies on its photophysical and photosensitive properties. In this study, formation of semiquinone anion radical and EA autosensitized photooxidation reactions were examined. Rate constant of semiquinone anion radical formation under anaerobic condition, and rate constant of autosensitized photooxidation reaction under aerobic condition were measured for Elsinochrome A (EA) by electron paramagnetic resonance and UV-visible spectra, respectively. The rate constant of formation was estimated at 0.767 min(-1) (R-2 = 0.963) using the zero-order reaction equation. The autosensitized photooxidation reaction occurred only when irradiation and oxygen co-existed in the EA solution. The average reaction rate constant was 3.22 +/- 0.36 (x10(-9), mol.s(-1)), and EA half-life was 4.48 +/- 0.52 (h) in the tetrachloromethane solution.