The following procedure was used to determine the effect of moisture on the adsorption of xenon from air onto activated carbon: A known amount of water is added to a sample of dried activated carbon and allowed to equilibrate. This activated carbon is then split between two cylindrical beds placed in a temperature controlled water bath, and air is passed through the beds sequentially. Because the beds contain pre-moistened activated carbon from the same sample, the first bed acts as a buffer, maintaining a constant humidity in the second. The mean holdup time of a pulse of133Xe injected into the second bed is used to determine the adsorption coefficient for xenon under these conditions. Measurements were made for three carbons activated to 35, 40, and 59%, respectively, at temperatures of 25°C and 55°C. The effect of moisture on the same activated carbon at these two temperatures shows an affine relationship that could be helpful in extending these results to other temperatures. At low moisture uptakes, a plot of the log(adsorption coefficient) vs. moisture uptake gives a straight line.