The results of density functional theory calculations on the structures, energetics, and hyperfine properties of radical H adducts to the C and S atoms of several thioketones and related species are presented. Despite the greater thermochemical stability of the C adduct in several cases, the isotropic proton couplings (and, where data are available, N-14 couplings) make it clear that the species observed in mu SR experiments are S adducts, counter to the assignments made by the experimentalists. The preference for S addition is probably a result of the barrier associated with the geometrical distortion required for addition at C or may indicate a formation mechanism involving a charged intermediate. The distribution of the magnitudes of the couplings of both C and S adducts shows that previous intuitive arguments are inadequate in the assignment of experimental data. Calculation of the dynamically averaged temperature-dependent coupling within a one-mode approximation corrected for zero-point bond stretching yields almost-quantitative agreement with experiment for thioacetamide assuming addition is to S.