We have shown that the addition of polyisoprene (PI) tethered chains to a polystyrene (PS) substrate produces a large increase in its adhesion to a PI cross-linked lens over the whole range of crack speeds (10(-10)-10(-7) m/s) that we have investigated. Such an increase can no the explained by the rate dependence of bulk viscoelastic losses and is believed to be due to the viscous pullout of the PI chains from the network At low crack speeds the fracture toughness Of the interface G(c) was found to increase linearly with velocity from a threshold value G0. At higher crack speeds a transition occurred, after which the fracture toughness increased at a much lower rate which could be attributed to viscoelastic bulk losses. The threshold toughness G0 increased monotonically with both the areal density of tethered chains and their degree of polymerization. The results are compared with existing models of interfacial failure by chain pullout. The measured rate of increase of G(c) with V implies there is a very high effective friction at very low pullout rates. This high friction is probably related to the very long time constant for a chain within a network.