Barriers and enthalpies for methyl radical addition to both the C- and S-centers of CH2=S, CH3CH=S, and (CH3)(2)C=S, and for the methyl-transfer reactions that interconvert the S-centered and C-centered radical products have been calculated via a variety of high-level ab initio molecular orbital procedures, including variants of the CBS, G3, G3-RAD, and W1 methods. An extensive assessment of the performance of the various theoretical procedures has been carried out. One of the important conclusions of this assessment is that the B3-LYP geometries, prescribed for several of these high-level composite methods, greatly overestimate the forming bond length in the addition transition structures, leading to a significant underestimation of the reaction barriers. The addition reactions are found to be highly exothermic and have relatively low barriers that are increased somewhat on methyl substitution. The reactions are also contra-thermodynamic-that is, despite a clear thermodynamic preference for the S-centered radical product, the barriers for the production of the C-centered radical via addition to S are lower. Interconversion of the C-centered and S-centered radical products via a methyl-transfer reaction is a high-energy process.