Molecules CH3X and free radicals (CH2X)-C-. can be deprotonated to form carbon-centered anions CH2X- and radical anions HCX.-, respectively. We have studied the geometric and thermochemical changes that accompany such deprotonation processes for a variety of substituents X including the pi-donor groups NH2, OH, OCH3, PH2, SH, F, Cl, and Br and the pi-acceptor groups BH2, AlH2, CHO, NO2, CN, and NC. Thermochemical properties calculated and discussed include the gas-phase acidities of the molecules and free radicals, the electron affinities of the (CH2X)-C-. free radicals, various dissociation energies, and the heats of formation of all species. The acidities of (CH2X)-C-. radicals are predicted to be greater than those of CH3X for pi-donor substituents but less for pi-acceptor substituents (except CN and NC). The changes that are predicted to occur upon deprotonation in C-X bond lengths, C-X homolytic and heterolytic bond dissociation enthalpies, C-H homolytic bond dissociation enthalpies, and radical stabilization energies may be understood by examining the orbital interactions that take place in each species.