When shear oscillations are performed with melts of polystyrene (PS)/poly(methylmethacrylate) (PMMA) blends the total deformation has a large elastic portion at low frequencies. This finding is caused by the interfacial tension acting between the two phases of the blends. A simple model allows one to describe the influence of the interfacial tension on the storage and the loss moduli in a broad frequency range for all mixing ratios of the blend components. The weighted relaxation spectra τH(τ) of such blends show an additional peak with a characteristic relaxation time that is correlated with the interfacial tension. From this characteristic relaxation time the interfacial tension α between the melts of PS and PMMA can be determined. The resulting α=(1.9±0.3)×10−3N/m at 170 °C is in good agreement with the value obtained from recovery following melt elongation of the blends.