In the frame of the law of the corresponding states, a systematic study of the influence on viscosity of the mass ratio in mixtures has been performed. To achieve such a goal, the viscosity of Lennard-Jones binary, ternary and 10-components mixtures in various thermodynamic states, and for various molar fractions, has been evaluated thanks to nonequilibrium molecular dynamics simulations. To focus on the mass ratio effect alone, the components in the Lermard-Jones mixtures differ only in mass. It is shown that none of the tested one-fluid models for mass is able to provide an accurate estimation of the mixture viscosities. It has been found that the mass of the pseudo-compound equivalent to the mixture in the one-fluid approximation for viscosity is density dependent and weakly temperature dependent. A purely empirical density dependent model (adjusted on equimolar results) is proposed. This model provides results consistent with direct simulations on mixtures as well as with results for the zero-density systems, the deviations being in all cases smaller than 3%. (c) 2005 Elsevier B.V. All rights reserved.