Because of the increasing emphasis on high-temperature processes and high-speed flight, the physical properties of gases at high temperatures and atmospheric pressures have acquired considerable importance. A recent study (1) of the viscosity of steam was carried out over the range of 450 to 1200 deg C in a heat-jacketed apparatus. At that point failure of refractory supports in the furnace precluded higher and lower temperature runs. It was decided to repeat this work with the same type of capillary efflux viscometer but a stronger support for the capillary, and also cover lower and higher temperatures. The viscosity of D2O vapor, not found in the literature, was also determined. In addition, the viscosity of argon was measured, in view of expressed doubts (2) as to the accuracy of Vasilesco’s results.