A theory-based model for liquid mixture viscosity is presented and evaluated using data from polymer-solvent systems. The model is based on the Eyring mixture viscosity model and the NRTL model for describing deviations from ideality. Comparisons of predicted and actual viscosity over the entire concentration range illustrate a significant improvement over the conventional liquid mixture viscosity model. The model is structurally capable of handling multicomponent mixtures and could be incorporated into process modeling and simulation software.