The viscosity of liquid alloys of Ag-In, Ag-Ge, Ag-Sn, Ag-Sb, and Au-Sn is analyzed in terms of a pseudo-potential method with special emphasis on its variation with the composition. The computational problems associated with the divergence of the key quantities of the theory are solved by presuming the oscillating part of the effective potential to decay exponentially with the distance. The disregard of the influence from neighbor atoms at distances of more than 10 times the hard core diameter is not believed to have an essential impact on the results. All systems under investigation show negative deviations from the additive law which complies well with the experimental findings. The predictions from a semi-empirical model based on simple physical quantities are also taken into consideration. The good overall agreement suggests that the semi-empirical model offers a useful, in many circumstances, more accessible alternative.