For Pd/MgO(100) pre-exposed to oxygen, the catalytic oxidation of CO and D-2, respectively, has been studied in the temperature range 100-300(degrees)C. At temperatures greater than or equal to 200(degrees)C, the CO2 desorption rate is independent of oxygen coverage, theta(O), and the reactive sticking coefficient for CO is close to unity. The D2O desorption rate is strongly dependent on theta(O). D-2 adsorption is blocked by adsorbed oxygen and the maximum D2O desorption rate is reached when almost all oxygen has been consumed (theta(O)< 0.03). The formation of an oxygen c(2x 2) structure, coexisting with the initial p(2x 2) phase, is reflected in the oxidation rates.