AES, XPS, TDS and LEED have been used to study the adsorption, the desorption and the reaction behavior of D atoms and CHx (1 ≤ x ≤ 3) species on the diamond C(111) surface. It is found that a small amount of chemisorbed D atoms can induce the (2 × 1) to (1 × 1) phase transition and hold the sp3 structure up to nearly 1400 K. The structural transformation can also be monitored by the optical second-harmonic generation technique. Thermal desorption of deuterium occurs mainly around 1300 K. Adsorption of CHx can also induce the phase change but not as readily as the D atoms. Annealing of the adlayer at a high temperature results in the partial conversion to graphitic carbon. D atoms can react with both the adsorbed CHx and the surface graphite. The surface bonding characteristics and the relevance of present results to the diamond CVD process are briefly discussed. © 1991.