Dissociative adsorption of borane on the Si(100) surface has been investigated with nonlocal density functional theory and a cluster model of the surface. The calculations predict that a Si-B bond is formed via a nucleophilic attack on boron, leaving BH2 and H fragments bound to the surface. The calculated reaction energy is -43.0 kcal/mol, and the reaction is unactivated. Comparisons and contrasts to calculated mechanisms of other adsorption reactions on the Si(100) surface are drawn.