The fracture toughness G(c) of interfaces between several styrene-based homopolymers was investigated by the asymmetric double cantilever bean method while the width of the same interfaces ai was investigated in parallel by neutron reflectivity. G(c) was found to be a direct function of a(i) provided that the molecular weights of the homopolymers were sufficiently high. The dependence of G(c) on a(i) could be divided in three regimes: a low toughness regime for a(i) < 6 nm, an intermediate regime for 6 nm < a(i) < 11 nm where G(c) increased steeply with a(i) and a plateau regime for a(i) >11 nm where G(c) was independent of a,. A molecular analysis of the transition from regime I to II showed that the average interpenetration distance of the homopolymers needed to activate the plastic deformation mechanisms was much less than the average distance between entanglement points in the bulk polymers. This result can be explained by the fact that a significant portion of the load-bearing strands may be loops rather than chain ends, so that an equivalent stress can be sustained by the interface with a much shorter interpenetration distance.