A model, which has previously been described for relating rheological behavior in highly polydisperse polymers to molecular structure, is shown to lead to a unique blending law. Unlike previously published polynomial blending laws in the open literature, no adjustable parameters or shift factors are necessary. The blending law suggested by the model is of the form
where ϕ is defined by the molecular weights of the components and the relaxation time, τ. Good agreement between predictions of this blending law and experimental results from the literature are demonstrated. This good agreement between the experimental results for blends of narrow molecular weight distribution samples and the predictions of blending law gives further credence to the previously reported relationships to describe the effects of polydis-persity on the rheological behavior. However, because of the complexity of the blending law in terms of the relaxation spectra, it is concluded that the description in terms of molecular weight is simpler and perhaps more fundamental.