The asymmetric double cantilever beam fracture test was used to measure the critical energy release rate, or fracture toughness, G(c), of an epoxy-polystyrene (PS) interface as a function of the grafting density, Sigma, and degree of polymerization, N, of carboxylic acid terminated deuterated polystyrene chains (dPS-COOH). The chain ends anchor to the epoxy, and their tails penetrate into the PS homopolymer. Forward recoil spectrometry (FRES) on the fracture surfaces provided a method to determine the total Sigma as well as the mechanism of interface failure. For N = 159 the grafted chains were too short to entangle and pull out of the PS, leading to no enhancement in G(c) over that of a bare interface. When the chain length was increased to N = 412 and 535, there was sufficient stress transfer to initiate crazes in the PS that break down by either disentanglement or scission of dPS-COOH chains in the craze fibrils. For long, well-entangled chains of N = 688, 838, and 1478 the chains broke near the epoxy-PS interface at low Sigma before significant crazing of the PS occurs. A transition from chain scission to crazing occurred at Sigma(c) = 0.03 chains/nm(2) for all three chain lengths, agreeing with experiments on diblock copolymer modified thermoplastic-thermoplastic interfaces’ and with the prediction from the fracture mechanism map(2) that the transition is independent of chain length. We observed a nearly linear decrease in the maximum achievable Sigma as N increased that can be explained in part by an entropic barrier that opposes the addition of new chains to the grafted brush. The toughest interfaces occurred with intermediate length grafted chains, N = 838, when the chains were well entangled and the grafted brush was dense enough, Sigma > Sigma(c), to cause energy dissipation through craze formation.