Adsorption isotherms have been measured on cleaned silver powder from 178 to 339°C at oxygen pressures of 0.226 Pa to 40 kPa using a vacuum ultramicrobalance. Adsorption equilibrium was found at all temperatures and pressures studied. The surface was prepared for the reproducible chemisorption studies and cleaned before the determination of each isotherm by an established method. Seven isotherms were measured that spanned the fraction of the surface covered ϑ from 0.17 to 1.0. The isosteric heat of adsorption q was determined by application of the the Clausius–Clapeyron equation at constant values of ϑ. From ϑ of 0.17 to 0.33, q decreases from 42 to 17.7 kcal/mol; it remains constant at 18.4±0.8 kcal/mol from ϑ of 0.33 to about 0.90, and then decreases to zero at the highest coverages and temperatures. By comparing the isosteric heat data with LEED and thermal desorption studies on Ag(111) by Rovida and coworkers, it is suggested the initial drop in q results from the formation of islands of surface silver oxide. The constant value is attributed to completion of the oxide layer and molecular adsorption on and/or through the oxide. The decrease in q at the highest coverages results from repulsions in the adlayer at T?275 and absorption at T≳302.