The photochemistry of chlorine dioxide, OClO, in amorphous ice has been investigated using FTIR and UV-vis spectroscopy. Exposure to ultraviolet light (lambda = 360 nm) quantitatively converted the OClO to the chlorine peroxy radical, ClOO. The asymmetric stretch of OClO at 1100 cm(-1) and the O-O stretch of ClOO at 1440 cm(-1), along with the characteristic UV absorptions at 360 and 260 nm, respectively, were used for monitoring the photochemical process, Under the dilute conditions used, the only photoproduct appears to be ClOO, in contrast with the gas-phase photolysis which yields predominantly ClO. These findings have implications for stratospheric ozone loss, Namely, the photochemical conversion of OClO to ClOO in stratospheric ice particles may represent a new ozone-depleting mechanism.