The reactions of H atoms with O-18 adsorbed on Cu(111) surfaces were studied as a function of the oxygen coverage between 80 and 600 K with Auger electron and thermal desorption spectroscopies. The formation of gaseous products was monitored simultaneously with admission of the atom flux. Adsorbed and gas-phase water were observed as products with temperature-dependent yields and oxygen was completely consumed during the reactions. At reaction temperatures below the onset of water desorption on Cu(111) near 145 K, adsorbed water was the prevailing product, monitored in postreaction desorption spectra. Above 150 K the kinetics of formation of gaseous water are in accordance with two consecutive reaction steps-formation of adsorbed OH followed by hydrogenation of OH toward water. which then desorbs isothermally. The rate-determining step for the water reaction is the formation of OH from gaseous H and adsorbed oxygen with a phenomenological cross-section of 0.26 Angstrom (2) at 200 K. At this temperature the OH hydrogenation cross-section is 5.8 Angstrom (2). Above 300 K a significant acceleration of the water rate was observed: This is interpreted through the action of an apparent activation barrier for O hydrogenation caused by the temperature-dependent availability of reactive O atoms.