Thin layers of a series of random PS/PMMS copolymers have been used to couple two sheets of PMMA, thereby permitting a study of the effect of interface width on interface toughness. The interface width and toughness varied with the copolymer composition. The use of a thin coupling layer between two PMMA sheets ensured that the crack tip craze occurred mainly in PMMA, so the material within which the crack tip deformation occurred did not change as the interface width was changed. The interface toughness was found to remain low for narrow interfaces until the interface width became larger than the mean entanglement spacing in the bulk. The toughness then climbs rapidly to saturation at an interface width of perhaps three times the entanglement spacing. A model is proposed here for the relation between interface width and interface toughness based on a well-tested model for interface toughness and known interface profile. However the assumptions made within the model, that entanglement density is constant through the interface and that the probability of a chain passing through the interface can be described just by a volume fraction ratio, are probably incorrect. The measured toughness is significantly less than that predicted at widths equal to and below the entanglement spacing. This disparity is probably caused by the entanglement spacing increasing over the bulk value in the area of the interface. For broad interfaces, where the entanglement density is expected to be close to bulk values, the predicted toughness agrees reasonably with experiment.