This paper reports experimental results for the viscosity of gaseous mixtures of HFC-125 (pentafluoroethane) + propane. The measurements were carried out with an oscillating-disk viscometer of the Maxwell type at temperatures from 298.15 to 423.15 K. The viscosity was measured for two mixtures containing 50.11 and 75.03 mol% HFC-125 in propane. The viscosity at normal pressure was analyzed with the extended law of corresponding states developed by Kestin et al., and the scaling parameters were obtained for unlike-pair interactions between HFC-125 and propane. The modified Enskog theory developed by Vesovic and Wakeham was applied to predict the viscosity for the binary gaseous mixtures under pressure. From comparisons between experimental results and calculated values of the HFC-125 + propane system, it should be concluded that the Vesovic-Wakeham method gives reliable predictions for the viscosity of a gaseous mixture containing both polar and nonpolar compounds.