Photochemical and thermal surface modifications of diamond powders and (100), (110), and (111) oriented single crystals were achieved. Clean, hydrogen-terminated diamond was chlorinated by heating in Cl-2(g) or by UV irradiation in Cl-2(g). The surface chlorine titer was shown to correlate with the dangling bond density on the single-crystal faces. Aminated diamond was produced by irradiating the chlorinated diamond in NH3(g). Surface nitrogen loading correlated with the dangling bond density. The structural features of the amine-terminated surface led to a decrease in the vibrational frequencies of the surface amine groups.