Several different effects of water in vacuum systems were studied. An outgassing rate measurement system was used to measure outgassing rates for different venting conditions. Both diffusion and desorption phenomena were observed. The drier the venting gas, the more significant the diffusion behavior. From the pressure buildup tests, it was shown that the increasing H2O partial pressure is much less than that of the other gases. More effective surface pumping for water vapor than for the other gases is thought to be the reason. For the outgassing behaviors mentioned, an effort was made to describe the phenomenon quantitatively by using simple models. In addition to the behavior in the static vacuum, water vapor also shows an interesting behavior in the dynamic vacuum. A few results will be discussed.