A simple liquid viscosity model for multicomponent mixtures containing polymers is presented. This model is essentially a new mixing rule for calculating the Newtonian viscosity of mixtures over the entire composition range using the pure-component viscosities. A modified MarkHouwink model is applied to calculate the Newtonian viscosity of pure polymer melts, and the Andrade/DIPPR correlation model is used to calculate the viscosity of pure conventional components. Two binary parameters, one symmetric and one antisymmetric, are introduced to capture nonideal mixing behavior of Newtonian viscosity in polymer solutions. Applications to polystyrene solutions and poly(ethylene glycol) solutions are presented.