The effects of molecular architecture on the fracture properties of semicrystalline polymers were probed at diblock copolymer-reinforced interfaces between polystyrene (PS) and polyethylene (PE). The PE used for this study was a model ethylene-butene copolymer which was chosen for its compatibility with hydrogenated 1,4-polybutadiene. This compatibility allowed the use of hydrogenated poly(styrene-b-1,4-tetradeuteriobutadiene) as the block copolymer. For a series of these diblock copolymers, the areal chain density (Sigma) and the molecular weight of the PE block (M-n) were varied systematically to observe their effects on the interfacial fracture energy (G(c)). At low Sigma, G(c) stayed relatively constant, and was roughly 1 J/m(2). Above a critical value of Sigma, the fracture energy climbed rapidly. This critical value decreased with increasing M-n. The detection of deuterium on the fracture surfaces indicated that pullout of the PE block was the predominant failure mechanism when M-n less than or equal to 30 kg/mol. Only when the molecular weight of the PE block reached 85 kg/mol was failure by chain scission observed. Since the entanglement molecular weight of PE is approximately 1 kg/mol, interfacial reinforcement does not appear to depend on the formation of entanglements for this system. The critical Mn coincides instead with the point at which the root-mean-square end-to-end length of the PE block exceeds the long period of the PE crystal lamellae (L). The preceding observation is consistent with the decrease in G(c) with increasing L near the critical molecular weight.