We have measured the fracture toughness, G(c), of an immiscible polymer/polymer [polystyrene (PS) and poly(2-vinylpyridine) (PVP)] interface reinforced with deuterium-labeled dPS-b-PVP diblock copolymers as a function of the number average molecular weight, <(M)over bar (n)>, of the polystyrene homopolymer, either monodisperse homopolymer PS (MPS) or polydisperse homopolymer PS (PPS). The dependence of G(c) on the PS molecular weight was investigated at different areal chain densities, Sigma of the copolymers. These values of Sigma were chosen to be in either one of two fracture mechanism regimes : chain scission or crazing. In the chain scission regime, G(c) is independent of the molecular weight of MPS and PPS. In contrast in the crazing regime (for Sigma less than or equal to Sigma(sat), where Sigma(sat) is the saturation Sigma for the copolymer at the interface), <(M)over bar (n)> of MPS has a strong effect on the fracture toughness. For this case, G(c) increases sharply around <(M)over bar (n)> x 100 000 and then levels off at higher <(M)over bar (n)> values. The polydisperse PS/PVP interface has a fracture toughness consistent with its <(M)over bar (n)> rather than its weight-average molecular weight, <(M)over bar (w)>. When the interface is covered with copolymer lamellae (Sigma much greater than Sigma(sat)), G(c) is found to be independent of <(M)over bar (n)>, of the MPS and is substantially larger than that for the PPS/PVP interface at the same Sigma. For the PPS/PVP interface, the low molecular weight portion of PPS swells the lamellar structure, resulting in a decrease in G(c) compared to that of the MPS/PVP interface. We have also measured G(c) as a function of composition of a blend of high and low <(M)over bar (n)> MPS, where <(M)over bar (n)> of the low molecular weight PS is below the entanglement molecular weight of PS. Dilution of the entanglement density of the homopolymer polystyrene results in a strong decrease in G(c). Our results are compared with recent models for craze failure. A continuum craze model using the full stress field proposed by Sha et al.(20) predicts the fracture toughness better than models(2,9) using the asymptotic stress field.