An increase in the PdO surface area by a factor of about 20 was observed after treatment of Pd foils in excess methane at 598 K. The surface area was increased as the O-2 partial pressure in the activation treatment was decreased. The activated foil was probed by the reaction of complete oxidation of methane in excess Oz. The rate was proportional to the PdO surface area and thus the turnover rate per unit of PdO area was the same as that for the untreated foil. Critical for this assessment was the determination of the PdO surface area measured by exchange of surface oxygen with O-18(2). The PdO surface area decreased after interaction with water when the sample was cooled. After treatment of the foil in excess methane, PdO but not Pd was identified by X-ray photoelectron spectroscopy, and carbon deposits from methane were identified on the surface and in the bulk by temperature-programmed desorption.