The e.s.r. spectra of methyl radicals produced by the ultraviolet irradiation of Tl(III) acetate show satellites attributed to simultaneous electron spin – nuclear spin transitions involving a proton of a neighboring water molecule. Measurement of the relative intensities of the satellites and the main transitions allows the average separation of the interacting proton from the unpaired electron to be calculated. Such measurements have been carried out over the temperature range −180 to −120 °C. The implications regarding the environment of the methyl radical in the ice matrix are briefly discussed.