Neutron reflectivity experiments were performed on bilayer samples to determine interfacial widths between poly(styrene-mn-methyl methacrylate) copolymers (P[S-ran-MMA]) and deuterated polystyrene (dPS) or deuterated poly(methyl methacrylate) (dPMMA) as a function of copolymer composition. The copolymer composition, f(s), varied from 0.49 to 0.85 mol% styrene. After a 4 h anneal at 150 degrees C, the interfacial width between the compositionally symmetric random copolymer (f(s) = 0.49) and dPS was smaller than with dPMMA. Equal interfacial widths for the copolymer with dPS and with dPMMA was found at [f(s)] approximate to 0.6. These reflectivity results are consistent with work by Kulasekere et al. and are discussed in relation to our previously published results on the fracture toughness of PS/PMMA joints reinforced with P[S-mn-MMA].