Joints of polystyrene (PS) and poly(methyl methacrylate) (PMMA) modified with similar to 50 nm of poly(styrene-co-methyl methacrylate) random copolymer [P(S-ran-MMA)] were investigated. Copolymers having styrene compositions of f(s) = 0.48 and f(s) = 0.73 were used. Transmission electron microscopy reveals that the copolymers phase separate to form a distinct layer at the joint such that there is an interface with each homopolymer. Interfacial fracture toughness measurements, using the asymmetric double cantilever beam geometry, show a strong effect of the PS to PMMA sheet thickness ratio; that is, the phase angle influences the measured interfacial toughness. Reflection infrared spectroscopy on fracture surfaces indicates that the crack propagates at or near the PS/copolymer interface for all thickness ratios and for both copolymers. In-plane crazing was not observed in front of the crack tip for these systems. Rather, strengthening appears to be exclusively a consequence of oblique crazes in the more compliant PS sheet which form at 45 degrees or 135 degrees relative to the crack direction. Joints modified with P(S-0.73-ran-MMA) exhibit denser oblique crazes than those modified with P(S-0.48-ran-MMA), resulting in a higher measured fracture toughness at all sheet thickness ratios or phase angles.