Atomic hydrogen (deuterium) adsorbed onto the diamond C(111) surface has been studied by infrared-visible sum-frequency-generation spectroscopy. Monohydride termination of H/C(111) is confirmed by the observation of sharp CH stretch (2838 cm-1) and CH bend (1331 cm-1) modes in the spectrum. Deuterium on the surface gives a CD stretch frequency of 2115 cm-1. The rate of hydrogen adsorption is measured in comparison with the rate of deuterium abstraction by hydrogen. Measurement of thermal desorption of hydrogen from C(111) suggests a near first-order desorption kinetics with an activation energy of 4.0±0.4 eV and a preexponential factor of 1015±2 s-1. On the bare reconstructed C(111) surface, distinct CC surface phonon features characteristic of the (2×1)-reconstructed surface are seen, which seem to support the modified Pandey model of Bechstedt and Reichardt.