Abstract: Based on Eyrings theory of rate processes, and taking into consideration the frequency of availability of new sites for passage of molecules from one equilibrium position to another, an equation relating viscosity of a binary mixture to its excess Gibbs free energy is The excess Gibbs free energy of activation G*E was found to have an optimum common ratio with respect to the corresponding excess Gibbs free energy GE. The frequency of availability of new sites is a function of composition and temperature. The calculated viscosity values for 10 binary systems (among which 3 are aqueous systems), cotnpared with 55 sets of corresponding data, were satisfactory, with a deviation range of RMS=0.002-0.09.