It is a very well-known experimental fact that the toughness of interfaces obtained by joining pairs of immiscible glassy polymers is strongly correlated to the interfacial width. Several models have been proposed in the literature to estimate the fracture energy of these interfaces, but the agreement displayed with the experimental data cannot be considered satisfactory. In this paper a new model is proposed for polymers with molecular weight higher than the critical value for the onset of entanglements. The model is based on a precise and realistic calculation of the areal density of entangled strands across the interface, that is the crucial parameter determining the toughness of the glassy joints. In this paper a new fracture regime is also introduced, called “partial crazing,” corresponding to a situation where, due to the fact that some of the load-bearing strands are broken during plastic deformation, the craze can start, but not fully develop. Model predictions are then compared with a series of literature fracture energy experimental data, showing excellent agreement.