Relative-rate experiments have been carried out, at several different temperatures between 240 and 340 K, to determine the Arrhenius rate parameters for the isomerization of the 1-pentoxyl, 2-pentoxyl, and 5-methyl-22-hexoxyl radicals. lsomerization rates were measured relative to the bimolecular reaction of each alkoxyl radical (RO.) with O-2. The order of reactivity, in terms of the rate of reaction for isomerization, was 5-methyl-2-hexoxyl > 1-pentoxyl > 2-pentoxyl, reflecting the relative-strengths of tertiary, secondary, and primary C-H bonds, i.e., the nature of the bond cleaved in the isomerization process. In addition to this, the measured Arrhenius activation barriers, and reported barriers for bimolecular reactions of the methoxyl radical with several simple hydrocarbons, are shown to correlate linearly with the bond strength of the cleaving C-H bond. The A-factors obtained for the three isomerization processes also scale linearly with the number of available abstractable H atoms, with a value of 3.0 x 10(10) s(-1) per H atom. This value combined with the activation barrier correlation constitutes a structure activity relationship for the estimation of the kinetics of the isomerization of simple alkoxyl radicals, at temperatures pertaining to the Earth's troposphere.