[en] Those cases were analyzed where the yield G(P) of scavenged electrons or holes per 100 eV of ionizing radiation absorbed follows the empirical formula G(P) = G/sub fi/ + G/sub gi/(α/sub s/c/sub s/)/sup 1/2/ [1 + (α/sub s/c/sub s/)/sup 1/2/]^-1, where c/sub s/ is the scavenger concentration, G/sub fi/ and G/sub gi/ are the free ion yield and geminate ion yield, respectively, and α/sub s/ is an empirical parameter, called the reactivity, which is characteristic of a given solvent-scavenger system. By combining this formula with the diffusion theory of Magee and Tayler, the following formula was obtained: α/sub s//k/sub s/ = (r/sub c/²/D) (G/sub fi//G/sub gi/)². Here k/sub s/ is the bimolecular rate constant for reaction of the electron or hole with the scavenger, D is the sum of the electron and hole diffusion coefficients, and r/sub c/ = e²/epsilon kT, where e is the charge of the electron, k is Boltzmann's constant, T is the absolute temperature and epsilon is the liquid dielectric constant. For electron scavengers in cyclohexane, n-hexane and isooctane, at T = 296⁰K, experimental values for α/sub s//k/sub s/ are known and are in satisfactory agreement with the ratio formula. For 14 other liquids the forumla predicts that α/sub s//k/sub s/ lies in the range of 0.04 to 155 ps. The formula also allows the temperature dependence of α/sub s//k/sub s/ to be assessed. For propane at T = 148, 183 and 230⁰K, α/sub s//k/sub s/ = 47, 0.7 and 0.1 ps, respectively. Although the ratio formula is specific to the representation of G(P) above, the method of derivation can be applied to any representation of G(P) so long as it is an analytic function of c/sub s//sup 1/2/