The properties of the interface between polystyrene (PS) and a random copolymer of polystyrene and poly(parahydroxystyrene) (PPHS) were modified by addition of a diblock copolymer of deuterated polystyrene (dPS) and poly(2-vinylpyridine) (PVP). Segregation of diblock copolymer chains to the interface between the immiscible blend components is driven by the compatibility of the dPS block with the PS phase and by a strong favorable interaction between the PVP block and the PPHS units of the random copolymer. Forward recoil spectrometry was used to measure the equilibrium excess of copolymer chains at the interface as a function of the copolymer volume fraction in the bulk PS phase. A dramatic increase in the interfacial adsorption is identified with the formation of emulsified "droplet" phases which appear when the interfacial free energy between the immiscible blends becomes vanishingly small. Dynamic secondary-ion mass spectrometry was used to characterize these emulsified phases, and to verify our picture of the copolymer adsorption process. Comparison to predictions from a detailed self-consistent-field theory gives an estimate for the magnitude of favorable interaction between PVP and the PS/PPHS copolymer (≅ 0.16kBT per repeat unit) and the magnitude of the interfacial tension between the two phases in the absence of a diblock copolymer (≅2 dyn/cm). © 1992 American Institute of Physics.