The relaxation of highly entangled bidisperse blends of polystyrene and bidisperse blends of polybutadiene were studied experimentally. The main objective was to quantify the effect that the interaction of short and long chains has on the relaxation of the blend. For the analysis of dynamic mechanical data, the simplest model for the relaxation time spectrum of the blend, H(lambda), was found to be satisfactory. It consists of a linear superposition H(lambda) = Sigma(i=1)(2)B(i)H(i)(lambda/A(i)) where the spectra of the nearly monodisperse components, H-i(lambda), were described by the BSW model. The interaction of the two blend components is expressed phenomenologically by the shift factors A(i) and B-i whose values arise naturally when fitting G’ and G" data of the blends. They depend on the weight fractions, w(i), and the molecular weights, M(i). The characteristic relaxation time of the short chains gets extended by the blending while the characteristic relaxation time of the long chain is reduced. The pattern in the shift factors suggests a blending rule for slightly polydisperse melts, i.e. for blends in which the molecular sizes are so close together that the characteristic time constants of the components are not noticeably affected by the blending.