Six model hydrogen transfer reactions, involving H atoms or methyl radicals with methane, ethene, and ethyne, were studied using ab initio quantum chemical methods in order to determine the level of theory at which activation barriers are accurately predicted. The calculations employed the following basis sets : 3-21G, DZP, DZP*, pVTZ, and pVTZ dagger. The quantum mechanical methods used were unrestricted Hartree-Fock theory and second- and fourth-order many-body perturbation theory. Spin contamination of the wave functions was removed using spin projection techniques. Activation energy barriers are predicted to within 2 kcal/mol of available experimental values for most reactions studied. Transition state structures and vibrational frequencies were also obtained and compared with other published ab initio results. Using the current ab initio results, transition state theory was used to calculate kinetic rate constants which compare favorably with experimentally measured results.