Gas viscosity is of fundamental importance in engineering calculations. In many cases either no data are available or they are outside of the temperature and pressure range of interest. Methods of estimating gas viscosities of halocarbon are available. However, when these correlations are used the results can deviate from experimental value by as much as 50%. This is not surprising since most of these interrelationships are based on a class of compounds which have physicochemical properties different from the halocarbons. We have devised, therefore, a method of estimating gas viscosities based exclusively on halocarbons.