The amount of hydrogen adsorbed on palladium during water formation has been studied by measurements of the rate at which hydrogen permeates a palladium membrane. The water formation and hydrogen permeation rates were measured simultaneously for palladium membranes exposed to a hydrogen-oxygen-argon mixture on one side and to pure argon on the other side. Investigations were carried out for the temperatures 100, 150 and 200 °C at a total pressure of 760 Torr. If the rates are plotted as a function of a, where a = pH(2)/(pH(2)+pO(2)), pH(2) and pO(2) are the pressures of hydrogen and oxygen at the palladium surface, respectively, it is found that, at 100 °C, a peak occurs in the water formation rate at amax˜0.2. The palladium surface is dominated by hydrogen for a>amax and is hydrogen deficient for a>amax. This is consistent with a rate limiting step for the water forming reaction where adsorbed hydrogen is one of the reactants. It is also concluded that the heat of adsorption for hydrogen on the palladium surface is significantly lower than previously derived from experiments made under ultrahigh vacuum conditions.