A quantum yield of ϕice=2.3¿10-4(¿4¿10-5) was determined for the photochemical degradation of 4-nitrophenol over the wavelength range of 300 to 370 nm in ice pellets (pH 5.6). Five reaction products were identified: hydroquinone, benzoquinone, 4-nitrosophenol, nitrate, nitrite, and the likely formation of organic polymers. The similarities of these results to the observations from 4-nitrophenol photolysis in aqueous solutions indicate that similar mechanisms are operative in both phases. The photochemical decomposition of 4-nitrophenol under conditions similar to those in polar snow and ice suggests that other direct and indirect photochemical process can occur in the polar ice/snow during spring and summer. ¿ 2000 American Geophysical Union