The adsorption and desorption phenomena of water molecules on stainless steel and aluminum surfaces have been studied by means of tritium tracer technique (TTT) with special emphasis on potential differences in the residence times in atmosphere and vacuum. Samples with a geometrical surface area of about 2 cm 2 have been exposed in an atmospheric environment loaded with a vapor pressure of HTO (tritium-labeled water) of known specific activity, and the adsorbed amount of water has been determined by liquid-scintillation counting technique. In previous studies it was found that, in atmosphere, adsorbed water quantities and their reproducibility depend significantly on the cleanliness of the surfaces. Furthermore, it was observed that desorption into atmosphere (or exchange with atmospheric humidity) is a rather slow process with a few percent of the adsorbed water molecules staying even for several days and with the water coverage versus time t characterized approximately by a 1/t(alpha) law with alpha approximate to 0.4. In this work, TTT measurements of water desorption have been extended into high vacuum. HTO-covered specimens have been transferred into a vacuum chamber soon after HTO exposure and have been kept under, high vacuum for a certain vacuum desorption time which. was varied from minutes to several days. It was found that water desorption from stainless steel and aluminum surfaces in vacuum is slower than in atmosphere with the residual coverage again described approximately by a 1/t(alpha) law but now with alpha approximate to 0.14 (stainless steel) and alpha approximate to 0.17 (A1). Water desorption rates calculated from coverage versus time are within the range of outgassing, data reported in literature, indicating that the TTT method can be a powerful tool for the characterization of potential new vacuum materials even if just small specimens are available. (C) 2007 American Vacuum Society.