A new model for the viscosity of miscible polymer blends is described. It combines a mixing rule for chain relaxation, based on the double reptation concept as adopted by Tsenoglou (1989), with a calculation of the concentration dependence of the monomeric friction factors, based on the self-concentration model of Lodge and McLeish (2000). The model is successful in anticipating the experimentally observed curvatures in plots of viscosity versus composition, and the predictions are in promising agreement with the data for several systems. The model introduces no freely adjustable parameters, and all the necessary parameters may be determined from measurements on pure components. (C) 2004 The Society, of Rheology.