A poly(styrene-g-ethylene oxide) copolymer with an average of 100 styrene units between the graft points is studied with respect to its interfacial activity in binary incompatible mixtures of polymers having different degrees of miscibility toward the graft copolymer components. The interfacial properties were studied by scanning electron microscopy, dynamic mechanical spectroscopy, and differential scanning calorimetry. The data obtained are compared with previously published data from a graft copolymer with the same composition but with shorter PEO branches and only 20 styrene units between them. In spite of their structure, the two graft copolymers could reduce the size of the dispersed phase in the case of athermal mixing. With a negative heat of mixing between only one graft component and the blend, the short branch graft had no effect, while the longer branch graft caused a reduction of the size of the dispersed phase. With a negative heat of mixing for the graft branches as well as for the backbone a strong compatibilizing effect was found for both types of graft copolymers. The results show that even very short parts of the backbone of a graft copolymer can contribute to compatibilization in a polymer blend, especially when the backbone has a negative heat of mixing with one of the blend components.