The Grunberg & Nissian equation with one parameter is widely recommended in the viscosity calculation. However, it is demonstrated that this equation fails to generate satisfactory results for size-asymmetric mixtures containing large and small molecules. In this work, a new one parameter viscosity model for binary mixtures has been developed on the basis of Eyring's absolute reaction rate theory and the Flory-Huggins equation. The concept of molecular surface fraction is introduced for modeling liquid mixture viscosities. The viscosity calculations of the new equation are compared with the Grunberg & Nissian equation for a broad range of chemical mixtures including 527 binary systems (containing 63 binary ionic liquid cosolvent systems) and total 17,268 viscosity points. The new equation was found to have an improved performance over the frequently employed Grunberg & Nissian equation, especially for size-asymmetric mixtures containing large and small molecules. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 517-524, 2011