Molecular dynamics simulations were used to study energy-dissipation mechanisms during the rupture of a thin adhesive bond formed by short chain molecules. The degree of dissipation and its velocity dependence varied with the state of the film. When the adhesive was in a liquid phase, dissipation was caused by viscous loss. In glassy films, dissipation occurred during a sequence of rapid structural rearrangements. Roughly equal amounts of energy were dissipated in each of three types of rapid motion : cavitation, plastic yield, and bridge rupture. These mechanisms have similarities to nucleation, plastic flow, and crazing in commercial polymeric adhesives.