The interaction of gas-phase atomic hydrogen with chemisorbed oxygen on Ru(001) has been investigated with high-resolution electron energy loss spectroscopy. As gas-phase hydrogen atoms impinge on the p(1X2) oxygen overlayer at surface temperatures below 100 K, water is formed, a reaction which has not been observed in previous studies of coadsorbed hydrogen and oxygen on Ru(001). The water molecules which are produced exist as adsorbed monomers initially but form clusters at greater extents of reaction. Since this reaction occurs at low surface temperatures, an Eley-Rideal-like mechanism is suggested in which hydrogen atoms from the gas phase react with adsorbed oxygen before being thermally accommodated to the surface. More than 50% of the saturation coverage of oxygen can be removed by means of gas-phase atomic hydrogen. Complete removal of the oxygen overlayer is inhibited by the water that is produced in the reaction.