This paper reports experimental results for the viscosity of gaseous mixtures of HFC-125 (pentafluoroethane) + HFC-134a (1,1,1,2-tetrafluoroethane) under pressure. 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 three mixtures (mole fraction of HFC-125 is 0.7510, 0.5001 or 0.2508). The viscosity at normal pressure was analyzed with the extended raw of corresponding states developed by Kestin et ai, and the scaling parameters were obtained for unlike-pair interactions between HFC-125 and HFC-134a. The modified Enskog theory developed by Vesovic and Wakeham was applied to predict the viscosity for the binary gaseous mixtures under pressure.