Electronic structure calculations have been performed on hydrogen-terminated carbon clusters. Optimized geometries of increasingly larger clusters were examined in order to obtain clusters that accurately model the 111 and 100 diamond surfaces. The clusters were used to characterize the energetics and transition-state structures for hydrogen abstraction by atomic hydrogen from the two surfaces. For the larger clusters, calculations at the MP2/6-31G*//HF/6-31G* level of theory were performed, and these were corrected on the basis of calculations using higher levels of theory applied to representative smaller clusters. It was determined that the barrier to H-atom abstraction from the 100 surface was 2-3 kcal.mol(-1) higher than that from the 111 surface. The reaction energy for abstraction was also found to be higher from the 100 surface than from the 111 surface, by 3-5 kcal.mol(-1).