The effect of polystyrene-block-poly(1,2-butadiene)-block-poly(methyl methacrylate) triblock copolymers (SBM) and their hydrogenated analogs polystyrene-block-poly(ethylene-co-butylene)-block-poly(methyl methacrylate) (SEBM) on the fracture toughness of the interface between immiscible polymers has been studied by using an asymmetric double cantilever beam fracture test. The fracture energy G(I) has been investigated as a function of the copolymer interfacial areal density Sigma and elastomer content as well as the temperature at which the samples were fractured. No reinforcement of the interface was found for a nide range of Sigma for all investigated triblock systems at room temperature. In fact, for a range of Sigma values the triblock was found to weaken, rather than strengthen, the interface. In addition, it was found that mixing just 25% of triblock into polystyrene-block-poly(methyl methacrylate) diblock copolymer was sufficient to remove the interfacial toughening effect of the diblock. The triblock copolymers were found to toughen, rather than weaken, the interface when the sample was fractured at a temperature below the glass transition temperature of the elastomer. These results are in agreement with a model that is based on the assumption that it is impossible to transfer enough stress across a plane glass-rubber interface to initiate crazing in the glassy polymers.