The concept of interpolymer complexation is applied to develop a new method for interfacial modification of immiscible binary homopolymer blends. In this method, end functional homopolymers terminated with acid and base groups are added to the blend to promote in situ end-complexation of the immiscible materials across the interface. More specifically, carboxy-terminated polybutadiene and amine-terminated poly(dimethylsiloxane) are employed to compatibilize an immiscible blend of polybutadiene and poly(dimethylsiloxane). The interpolymer complex that forms between acid and base groups creates a "block-copolymer-like" structure that spans across the interface. Pendant drop tensiometry is applied to measure the interfacial tension reduction afforded by this complexation. The interfacial tension data show behavior similar to that observed for block copolymer addition to homopolymer blends : there is initially a linear decrease of interfacial tension with the concentration of functional homopolymer up to a critical concentration at which the interfacial tension becomes invariant to further increases in the concentration of functional material. In contrast to the behavior of block copolymers, however, the formation of interpolymer complexes is dependent on the equilibrium between associated and dissociated functional groups. That is, the ultimate plateau value for interfacial tension reduction is dependent on the equilibrium between associated and dissociated functional groups. That is, the ultimate plateau value for interfacial tension reduction is dependent on the functional group stoichiometry. A reaction model for end-complexation is developed including the effects of carboxylic acid dimerization in order to reproduce the interfacial tension reduction data. Fourier transform infrared spectroscopy is applied to determine the appropriate rate constants. The model provides a reasonable qualitative description of the interfacial tension results but cannot quantitatively predict the critical compositions observed experimentally. Finally, it is shown that the overall interfacial tension behavior is consistent with that predicted by current statistical thermodynamic theories developed for block copolymers.