Field experiments in Ny-¿lesund, Spitsbergen Island (Svalbard) demonstrated that water ice can be a suitable reaction medium for photochemical transformations of organic pollutants. Several aromatic carbonyl, chloro, nitro or hydroxy compounds, frozen in the ice-matrix samples, underwent very efficient sunlight-induced chemical changes. The photoproducts, in many cases completely different from those obtained from liquid solution photolysis, might pose a high toxicological risk to biota when they enter the environment. It is concluded that the results could be applicable to natural snow and ice, and that possible photochemical transformations should be considered in the ice-core record studies.