[en] The effect of added solutes on the decay kinetics of primary positive ions in irradiated cyclohexane has been studied using the microwave absorption method. Absolute rate constants, in the range 1 to 3.5 x 10¹¹ M^-1s^-1, have been determined for the reaction of positive ions with ethanol, ammonia, cyclopropane, benzene, dimethylaniline and biphenyl. For all of the above solutes the activation energy for reaction is found to be less than 1 kcal/mole. The rate constants obtained are compared with reactivity parameters available from steady-state radiolysis experiments. Taking for the yield of geminate ions a value of 5.0 (100 eV)^-1, the characteristic geminate ion lifetime, lambda^-1, in pure cyclohexane is estimated to be 2.2+-0.5 ps. (Auth.)