Potential energy profiles for the insertion of gas phase C atoms, and CH, CH2, C-2, CA and C-3 radicals, into C-H and C-C bonds on a 2 x 1 reconstructed, H-terminated diamond (100) surface have been explored using both quantum mechanical (density functional theory) and hybrid quantum mechanical/molecular mechanical (QM/MM) methods. Both sets of calculations return minimum energy pathways for inserting a C atom, or a CH(X), C-2(X), or CH2(a) radical into a surface C-H bond that are essentially barrierless, whereas the barriers to inserting any of the investigated species into a surface C-C bond are prohibitively large. Reactivity at the diamond surface thus parallels behavior noted previously with alkanes, whereby reactant species that present both a filled sigma orbital and an empty p(pi) orbital insert readily into C-H bonds. Most carbon atoms on the growing diamond surface under typical chemical vapor deposition conditions are H-terminated. The present calculations thus suggest that insertion reactions, particularly reactions involving C(P-3) atoms, could make a significant contribution to the renucleation and growth of ultrananocrystalline diamond (UNCD) films.