Kinetically and thermodynamically plausible reactions that can explain the growth of diamond from acetylene are introduced. This class of reactions includes the addition of acetylene to a biradical with at least one of its sites being either a (100)-(2X1) dimer carbon or a secondary carbon, so that the admolecule can undergo a series of H-additions, H-abstractions, and beta-scissions leading to incorporation of its carbon atoms into a diamond lattice. Several examples are presented and supported with the results of semiempirical quantum-chemical and transition-state-theory calculations. Among those considered, the most favorable reaction pathway is initiated by the addition of an acetylene molecule that forms a bridge between adjacent dimer rows.