The correlation between interfacial width and adhesion, as expressed by the fracture toughness of the interface, in glassy polymers is investigated. The interfacial width is measured accurately by neutron reflectivity, and the fracture toughness of the interfaces is determined from a double cantilever beam test. Two experimental situations are considered : the case of an interface between two immiscible polymers, polystyrene (PS) and poly(p-methylstyrene) (PpMS), and the case of a PS-PS interface, where the interfacial width and the fracture toughness are measured for different interdiffusion times. In both cases, a direct correlation between the fracture toughness and the interfacial width is found. After an initial rapid increase in toughness due presumably to chain-end diffusion, most of the subsequent increase in toughness occurs over a relatively narrow range of interfacial widths between 9 and 12 nm. The fracture toughness stays constant with further interdiffusion. The results are consistent with respect to a model where formation of entanglements is the determining factor for the development of adhesive strength at the interface.