The influence of enthalpic interactions on interfacial adhesion between immiscible polymer matrices and reinforcing block copolymer segments has been studied using the transmission electron microscopic (TEM) methodology of Creton et al. We examined the behaviour of four statistical styrene-acrylonitrile (SAN) copolymers, each having different acrylonitrile (AN) content, blended with polystyrene (PS) as the minor component, and reinforced by three poly(methyl methacrylate-b-styrene) (PMMA-b-PS) block copolymers of differing molar masses, viz. 20 000, 65 000 and 680 000 g mol-1. These observations were compared with similar experiments on poly(methyl methacrylate) (PM MA) blended with PS and reinforced by PMMA-b-PS. Emulsification was observed with all three PMMA-b-PS copolymers. Crazes were formed in the SAN matrices and a statistical evaluation of interfacial failures was performed on the discrete PS domains that lay within the crazes. For the two block copolymers of higher molar mass, optimal reinforcement of the interfaces was observed independent of the SAN composition. With the 20 000 block copolymer, however, the pattern of the interfacial failure depended strongly on the SAN composition. Specifically, it was observed that the fraction of the discrete particles that suffered interfacial failure, and led to the creation of large voids in the crazes in these blends, increased with increased AN content of the SAN matrix. Thus, we found that the fraction of discrete PS particles that produce large voids in crazes of blends containing SAN33 is always higher than in blends containing SAN15, when reinforced with the 20000 PMMA-b-PS. We infer that the critical molar mass required of a mechanically reinforcing copolymer depends on the short-range (attractive and repulsive) interactions between the blend components in the interfacial region. The TEM method could not, however, distinguish between reinforced and neat PMMA/PS blends, all of which showed strong adhesion. This is attributed to the comparatively diffuse interface in the PMMA/PS system, a consequence of the relatively weak repulsion between these two polymers.