An asymmetric double cantilever beam (ADCB) was utilized to determine the ability of a series of styrene and methyl methacrylate copolymers with varying architectures to compatibilize the polystyrene/poly(methyl methacrylate) interface. Diblock, triblock, pentablock, and heptablock multiblock copolymers with similar molecular weights were compared to a random copolymer. When the surface is saturated with copolymer, PS/PMMA interfaces compatibilized with pentablock copolymers [S-M-S-M-S(30) and NI-S-M-S-M(30)] were the strongest, followed by triblock [S-M-S(50) and NI-S-N,1(50)] and then diblock [S-M(100)]. The least blocky structures, heptablocks [S-AT-S-M-S-M-S(21) and M-S-M-S-M-S(21)] and random, provided the weakest interfaces under similar conditions. The ability of the multiblock copolymers to strengthen the PS/PMMA interfaces was attributed to multiple interface crossings and blocks of monomers that are able to anchor into the homopolymers. The results suggest that block lengths with molecular weight greater than 21 000 are required for adequate anchoring into the PS/PMMA homopolymer phases. Surprisingly, a dependence of the interfacial fracture toughness on copolymer composition was not observed for the multiblock copolymers studied. Both styrene-centered and methyl methacrylate-centered multiblock copolymers gave comparable results even though the compositions of the comparable copolymers differed greatly, as much as 70/30 to 30/70. In addition, increasing block lengths in multiblock copolymers of a given architecture increased interfacial adhesion.