Radiation-induced hydrogen exchange has been observed in binary mixtures of water with 1,4-dioxan (up to 11 mol dm-3 dioxan) and with t-butyl alcohol (up to 10 mol dm-3 t-butyl alcohol). In both cases, the rate plateau observed over the region ∼1-3 mol dm-3 is followed at higher concentrations by a steeper dependence of exchange rate on concentration of the organic component. Electron-scavenging (by nitrate ion) is found to be effective even at a high concentration of t-butyl alcohol. It is con sidered that the exchange mechanism in the high-concentration region must still follow the steps RH → R· → R- → RT, but that the generation of R· and esolv- in part arises from interaction between the radiation and RH, whereas in dilute aqueous solution the process of energy deposition concerns only water molecules. The rate increase at high concentrations is probably attributable to the increased importance of the intra-spur reaction between R· and esolv- in largely non-aqueous media. In the ternary systems water + dioxan + cyclohexane-1,2-diol and water + dioxan + cyclohexane, exchange is observable in both non-aqueous components of the mixture. The latter example confirms our conclusion that radiation-induced aliphatic hydrogen exchange is a process which occurs in all irradiated aqueous solutions of aliphatic substrates, including saturated hydrocarbons.