Welding experiments using a double cantilever beam test geometry for a miscible poly(styrene-co-acrylonitrile)/poly(methyl methacrylate) (SAN/PMMA) system showed the expected linear increase of the fracture toughness, G(a), with the root of annealing time. For immiscible pairs of PMMA and SAN it was found that G(a) increases linearly with the equilibrium interfacial thickness or, equivalently, to chi-1/2, chi being the Flory interaction parameter.