Interfacial tension is a very important material parameter in two-phase polymer blends. It determines the morphology development during processing, which is crucial for the end-use properties of the material. Although different techniques for interfacial tension measurement give comparable results for immiscible polymers, the determination of the interfacial tension in lower critical solution temperature blends is not straightforward. This is illustrated for poly(alpha-methyl styrene acrylonitrile)/poly(methyl methacrylate)(PalphaMSAN/PMMA), a slightly incompatible polymer pair. Interfacial tension has been measured with three different techniques: small-amplitude oscillatory shear, recovery after elongation, and elongation of a multilayer sample. The large differences in these results can be attributed to the fact that most experimental techniques determine an apparent value, rather than the thermodynamic equilibrium value, of the interfacial tension. The latter is only obtained if the measurement is performed under quiescent conditions on a system that is composed of the coexisting PalphaMSAN-rich and PMMA-rich phases. The apparent interfacial tension depends on the actual composition of the phases and on the deformation of the interface. An order of magnitude approximation for such effects has been derived from theoretical considerations. Finally, each of these apparent values can be of practical importance. If a blend is prepared by melt mixing of the pure polymers, a high apparent value of interfacial tension should be considered. If, however, a blend is prepared by phase separation of a homogeneous mixture, the thermodynamic value is important.