The kinetic method has been applied to the determination of proton affinities (PAs) of free radicals. Using this method, a value of 205.6 +/- 0.3 kcal/mol for the PA of the phenoxyl radical is obtained. This PA is in good agreement with both a value of 204.4 +/- 2.0 kcal/mol, determined by FT-ICR bracketing experiments, and a value of 204.8 +/- 2.0 kcal/mol, derived from published experimental data, by use of a thermochemical cycle. The PAs of several substituted phenoxyl radicals were also determined by the kinetic method. In addition, the ability of the method to determine the PAs of the isomeric radicals, methylphenoxyl and hydroxybenzyl, each derived from o-, m-, and p-cresols, is demonstrated. The PAs determined for the isomeric p-methylphenoxyl and p-hydroxybenzyl radicals, which were found to be respectively 5.1 and 9.0 kcal/mol higher than the PA of phenoxyl radical, were supported by the results of ab initio PUMP2 calculations, performed with the 6-31G(**) basis set. The calculations also found that the p-cresol radical cation formed by protonation of either of these isomeric radicals is the thermodynamically preferred product.