Mineral oil viscosities are important in engineering of petrochemical processes and coal conversion. There are problems in experimental determination in the range of extreme process conditions characterized by high temperature, high pressure, vaporization and gas solution. Additional calculation are choosed in order to Calculated reduce expense. On the basis only of pressure and pared. few analytical data a way is shown to predict liquid viscosities of mineral oils at process conditions. Phase equilibria reflections are used to take into consideration the effects of vaporization and gas solution. Calculated results and experimental data are compared.