Ab initio calculations were performed on prototypical intramolecular H atom transfer reactions in alkyl radicals, namely, the identity reactions of 1,2-, 1,3-, 1,4-, and 1,5 intramolecular isomerizations in the ethyl, 1-propyl, 1-butyl, and 1-pentyl radicals, respectively. The equilibrium geometries and the transition structures have been optimized at the UHF/6-31G* and/or UMP2/6-311G** levels. The activation energies at 0 K were calculated with the MP-SAC2 and BAC-MP4 methods. The computed barrier heights (41.1, 41.6, 24.6, 17.2 kcal mol(-1) at the MP-SAC2 and 43.1, 41.0, 25.1, and 18.8 kcal mol(-1) at the BAC-MP4 level of theory) follow the trend expected and are in reasonable agreement with experimental data on related reactions. The differences between the ab initio barrier heights of alkyl isomerizations and that of the corresponding bimolecular reaction are in good agreement with the strain energies of the corresponding cycloalkanes.