The effects of protonation on the geometries and stabilization energies of prototypical •CH2X radicals (X = NH2, OH, OCH3, PH2, SH, F, Cl, Br, CN, CHO, and NO2) have been studied with the use of ab initio molecular orbital calculations at the G2 level. The proton affinities at X of the •CH2X radicals and the analogous substituted methanes, CH3X, are compared and the corresponding heats of formation calculated. For π-donor substituents (X = NH2, OH, OCH3, PH2, SH, F, Cl and Br), protonation at X leads to considerable +C :XH character for both CH3XH+ and •CH2XH+, resulting in substantially lower heterolytic bond dissociation enthalpies and longer C−X bonds. Protonation also strengthens the C−H bonds in CH3XH+ and, in combination with the reduced interaction of the lone pair on X with the singly-occupied orbital at the radical center in the radicals, results in negative radical stabilization energies for •CH2XH+. For the π-acceptor substituents (CN, CHO, and NO2), protonation enhances hyperconjugative electr...