A change in regioselectivity has been observed in the hydrogen atom transfer (HAT) reactions from 4-alkyl-N,N-dimethylbenzylamines (alkyl = ethyl, isopropyl, and benzyl) to the phthalimide N-oxyl radical (PINO) by effect of protonation. This result can be rationalized on the basis of an acid-induced deactivation of the C-H bonds α to nitrogen toward HAT to PINO as evidenced by the 104-107-fold decrease in the HAT rate constants in acetonitrile following addition of 0.1 M HClO4. This acid-induced change in regioselectivity has been successfully applied for selective functionalization of the less activated benzylic C-H bonds para to the CH2N(CH3)2 group in the aerobic oxidation of 4-alkyl-N,N-dimethylbenzylamines catalyzed by N-hydroxyphthalimide in acetic acid.