An apparatus is described to measure volumetrically the amount of gas adsorbed by a catalyst at constant pressure and at exposure times as short as 0.1 min. The volumes of oxygen adsorbed by a technical silver catalyst at pressures of 200, 400, and 700 mm. and in the temperature range of 180° to 300°C. have been measured. Surface area determinations of 0.33 to 0.38 sq. meter per gm. of catalyst by physical adsorption and chemisorption methods show that each silver atom of the surface is associated with approximately one oxygen adatom at a monolayer coverage. The initial rate of adsorption of oxygen is directly proportional to its pressure. Observed transitions in the oxygen adsorption rates indicate that more than one type of oxygen complex occurs on the catalyst surface. The Elovich equation provides the best approximation of the adsorption rate data but is of limited applicability in interpretation of the mechanism of adsorption. Values of 22–29 kcal./mole and 17–25 kcal./mole have been calculated for the apparent activation energy and isosteric heat of adsorption respectively.