Pumping times after air exposure with their strong implications on the economy of vacuum processes are mainly caused by the desorption of rather tiny quantities of water in vacuum systems. There is no doubt that, by reducing the adsorbed quantity or by shortening its sojourn time, faster pumping cycles could be achieved. The slow progress in this field is, among others, due to the lack of simple and quantitative methods for measurements of atmospheric water coverage and its kinetics. In order to find a method that exhibits the same sensitivity and accuracy in atmosphere as well as in vacuum, radioactive tracer technique has been studied with particular emphasis on tritium-tracer-technique (TTT). This method allows precise measurements of low coverage (down to 1 percent of a "monolayer" even on rather small samples) and of its kinetics over periods of several days. In addition, water coverage can be measured in a way that is not effected by other surface contaminants, or by shortcomings of usual vacuum gauges (sensitivity drift, outgassing, gas reactions on hot filaments, etc). Hitherto, atmospheric water adsorption on square-cm-sized samples of stainless steel, aluminum, silicone, gold, and glass has been studied. It was found that water adsorption depends highly on the cleanliness of the surface with no significant difference between stainless steel and gold. Furthermore, it turned out that sojourn times in vacuum are at least equal to or even much longer than those in atmosphere. Basic aspects of the method as well as experimental data will be discussed.