The effects of UV irradiation, both in the presence and absence of hydrogen peroxide, as well as of gamma irradiation on 2- and 4-chlorophenol in a solid water ice matrix have been studied and compared to those effects known to occur in aqueous solutions. While UV photolysis (>280 nm) of monochlorophenols leads to efficient coupling reactions in ice and photosolvolysis products in liquid water; hydroxylation to chlorobenzenediols is the main pathway in the presence of H₂O₂ in both phases. The results show that the solute molecules accumulate in a layer surrounding the ice crystal walls during the freezing process, where they then react. The radiation chemistry of chlorophenol ice samples involves preferential coupling reactions at −78 °C rather than reactions with the OH radicals produced by cleavage of water molecules under the conditions employed (1 kGy h⁻¹). The apparent similarities between the chemistry in the UV/H₂O₂-treated liquid and solid, and γ-irradiated liquid and solid samples are discussed. It is suggested that the reactions of OH radicals within polycrystalline ice or snow are important natural processes that should be considered in environmental, ice-core or astrophysical research.