The quantum efficiency of chlorpromazine hydrochloride (CLP HCI) at 253.5 m μ in aqueous media (pH 1.5) was 0.18 and 0.14 under normal and oxygen-free atmospheres, respectively. The quantum efficiency remained constant within the concentration range and periods of irradiation studied, but increased at pH values above 4.5. The photoreaction appears to follow zero-order kinetics. Reactivity toward acetic anhydride, partition studies, and infrared spectra of light degraded solutions of CLP HCI suggest ring hydroxylation occurs during irradiation. Spectral evidence indicated the formation of a semiquinone free radical intermediate on irradiation of aqueous solutions of CLP HCI at 253.5 m μ . Chlorpromazine-5-oxide hydrochloride (CLP-O HCI) forms by disproportionation of this free radical in aqueous media, both in the presence and absence of dissolved oxygen. The phenolic and other unidentified compounds formed on irradiation of CLP HCI apparently result from degradation of the photolabile CLP-O formed.