Chemical reactions of acetylene on diamond (100) surfaces were investigated theoretically. Quantum-mechanical calculations at semiempirical and ab initio levels were performed on a series of small- and large-size clusters to explore possible reaction steps responsible for diamond growth at conditions typical of chemical vapor deposition. Several new reaction pathways were identified for the growth of diamond from acetylene. Surface migration and etching of a > C = CH2 adspecies were examined, and the former was proven to be Isolated dimers on the diamond surface were found to be kinetically unstable and to convert rapidly into > C = CH2 groups. The resulting mechanistic picture explains the experimentally observed pattern of diamond growth through the formation of dimer-row domains. The results provide further evidence for the critical role of surface diffusion in the chemical vapor deposition of diamond.