A free-volume and friction viscosity model is presented versus pressure and temperature, valid for both gaseous and dense fluids. This model involves only three adjustable parameters for each pure compound. It is able to represent the gas-liquid transition and the behavior in the supercritical conditions. The model has been successfully applied to methane (885 data points for 0.01< or =P< or =200 MPa and 90.7< or =T< or =600 K) and to propane (1085 data points for 0.01< or =P< or =200 MPa and 90< or =T< or =600 K) in the gaseous and dense states (average absolute deviation is 2.59% for methane and 2.50% for propane, with maximum deviation of 14.8% for methane and 9.19% for propane). It has also been applied to hexane, octane, dodecane, benzene, trans-decaline, and 2,2-dimethylpropane (903 data points) in a large pressure range (up to 505.5 MPa). Considering these compounds the maximum deviation is 19.5% (for octane) and the average deviation is 3.51% in the worst case (dodecane, which has data points up to 501.6 MPa).

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