We have shown that the addition of small amounts of carbon black can drastically reduce the interfacial fracture toughness between polymers. This is interpreted as being due to strong interactions between the filler particles and the polymer chains which hinder interfacial adhesion. The effect is less severe when graphitic carbon black particles are used. The fracture toughness was found to increase with time as t(1/2), regardless of filler concentration, indicating that interface formation was diffusion limited. A distinct minimum with filler concentration in the fracture toughness of interfaces annealed for times longer than 5 min was found. This feature could be explained as a balance between the increase in modulus and the decrease in polymer chain dynamics as a function of carbon black concentration. The addition of colloidal silica, where the surface interactions are screened, was found to reinforce the interface, as predicted by the Guth-Gold relationship. Mixing small quantities (<2%) of inert fillers with interfacially active ones restored the fracture toughness. This observation has practical importance since one can now obtain optimum adhesion without compromising the mechanical integrity provided by the reactive filler.