The segregation behavior of a small molecule plasticizer additive at an immiscible polymer-polymer interface was explored. Deuterated and nondeuterated plasticizer, benzylbutyl phthalate, were added to bilayer and multilayer thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA), and the resulting interfacial structure was analyzed by direct and indirect depth profiling techniques, using nuclear reaction analysis and neutron reflectometry, respectively, In the latter, contrast matching of the two polymer components together with a multilayer sample was used to enhance the scattering cross-section depth profile due to small molecule interfacial segregation and thus provide greater sensitivity to small amounts of interfacial segregation. The results indicated a larger-than-expected Segregation of the additive to the polymer interface and consequently a greater degree of compatibilization of the PS-PMMA interface.