Schallamach's theory of rate-dependent bond dissociation is used to understand the fracture of a polymer-glass interface. The model system consists of an elastomeric film (20 mu m thick) of poly(dimethylsiloxane), chemically bonded to a silanized glass substrate. The fracture energy of the interface varies logarithmically with velocity, which is consistent with Schallamach's theory of forced bond scission. The activation energy (151 kJ/mol) of siloxane bond scission, as inferred from this study, is similar to that (147-180 kJ/mol) obtained from thermal de-polymerization and stress relaxation kinetics of the siloxane polymers. It is shown that the equilibrium threshold toughness of an interface is simply the product of the areal density of the polymer chains and the energy to dissociate a single bond. The well-known Lake-Thomas amplification of fracture energy by the number of bonds per chain can be understood only on the basis of the nonequilibrium aspects of the bond dissociation phenomena.