The fracture toughness G(c) of epoxy/high-impact polystyrene (HIPS) interfaces was measured as a function of grafting chain density Sigma of carboxylic acid terminated deuterated polystyrene (dPS-COOH) chains of various degrees of polymerization N. The dPS chains penetrate into the bulk HIPS whereas the - COOH end functional group is chemically bonded to the epoxy. For short chains, e.g., N = 160, no effective entanglements can be formed between the dPS chains and the PS matrix of the HIPS, and thus no enhancement in G(c) over that of a bare interface. For longer chains, N = 410, the interface fails by chain pullout from the PS matrix of HIPS at low Sigma. There is a transition from chain pullout to crazing above an areal chain density Sigma(+) approximate to 0.025 chains/nm(2) at this chain length. For very long chains, e.g., N = 1860, even though each chain is well entangled, the maximum grafting density achievable is very low and such an interface fails by scission of the chains so that the interface fracture toughness is also low. Large values of G(c) are observed at intermediate-chains lengths where both effective entanglements can be formed and a large Sigma can be achieved. Under these conditions, the interface fails initially due to the formation of crazes in the HIPS side of the interface and the subsequent breakdown of one of these crazes at the interface. As Sigma increases, the mechanism of interface failure undergoes a transition from scission of the bridging chains before craze formation to a mechanism where crazing occurs and is followed by craze failure. The critical areal chain density at which this transition occurs, Sigma(c), is independent of N and is found to be similar to 0.015 chains/nm(2) for the epoxy/HIPS system. The maximum grafting density achievable is observed to decrease linearly with increasing N, and the optimum interface adhesion appears to be achieved with N around 1000. The results are compared with those of epoxy/PS interfaces for which the PS has a higher crazing stress and thus a higher Sigma(c) similar to 0.03 chains/nm(2).