A general kinetic treatment applicable to ionic and free radical reactions in the radiation chemistry of liquids is presented. From the simple theory of homogeneous kinetics, suitable expressions are derived for the concentrations and lifetimes of ions and free radicals in the steady state. The problem of geminate recombination of ions is also discussed. For chain reactions, the predominant contribution to the kinetic chain length comes from the participation of free ions and free radicals undergoing bimolecular recombination, since the mean lifetime of the intermediate under these conditions is considerably greater than for the corresponding species which undergo cage or geminate recombination. The practical problem encountered in many studies of radiation-induced chain reactions is to achieve the limiting rates of propagation which apply only in the absence of chain termination by impurities or products of the radiolysis. This problem is discussed in connection with the free radical isomerization of 1-bromobutane and the ionic polymerization of isobutylene. Through the use of stringent drying conditions, G (-m) values as high as 108 have been obtained in the latter case. The kinetic data can be used to calculate the propagation rate constant for the cationic polymerization of isobutylene. It is possible to predict the maximum rates and G-values which are obtainable as a function of dose rate in the temperature range over which these reactions have been studied.