Diffusion of water vapor was measured at 25 degrees C in the binary systems water vapor-He and water vapor-N2 at approximately 1, 4 10, 20 and 50 atm. Diffusion was also measured in the ternary system water vapor-He-O2 in both 2%O2-98%He, and at 1 and 20 atm. At 1 atm, comparison of binary diffusion coefficients (delta) shows that water vapor diffuses 3.3 times more readily in He than in N2. Values of delta at elevated pressures were reduced in proportion to 1/p, the absolute pressure, in qualitative agreement with Chapman-Enskog theory. However, the reduction in Delta was less than predicted at pressure above 4 atm in water vapor-N2 and above 20 atm in water vapor-He. The deviations from theory are more pronounced in N2 than in He, and become larger at higher pressures. When 26% O2 is present in the ternary system water vapor-He-O2, diffusion of water vapor is reduced to about 60% of its rate in pure He. In contrast, 2% O2 in He has little effect on diffusion of water vapor.