Based on the similarity between P-upsilon-T and T-mu-P relationships, viscosity (mu) models for pure hydrocarbons (mainly for n-alkanes) have been developed from the three-parameter Patel-Teja (PT) and two-parameter Peng-Robinson (PR) cubic equations of state. The EOS-based viscosity models are applicable to both liquid and gas phases? and have been successfully extended to hydrocarbon mixtures containing defined components as well as undefined C-7(+)-fraction. Compared to the typical viscosity correlations widely used in the petroleum industry, significant improvement in prediction accuracy was achieved. Since the P-upsilon-T, V-L-E and viscosity calculations can be performed by using a single equation of state, the thermodynamic consistency of reservoir simulation and process simulation calculations is improved.