Using molecular-beam relaxation techniques and isotopic exchange experiments, the water-formation reaction on Pd(111) has been shown to proceed via a Langmuir-Hinshelwood mechanism. The reaction product H2O is emitted from the surface with a cosine distribution. The rate-determining step is the formation of OHad in the reaction Oad + Had → OHad. The activation energy for this step is 7 kcal/mole with a pre-exponential factor, v, of 4 × 10-8 cm2 atom-1 sec-1. This value for v lies well below that observed for simple second-order desorption of dissociatively adsorbed diatomic gases, but is roughly of the order of that obtained for the oxidation of CO on Pd(111). The formation of H2O proceeds differently under conditions of excess O2 or H2. In an excess of H2, the kinetics is dominated by the transport of atomic hydrogen between the bulk and the surface as was found for the H-D exchange reaction on Pd(111). In an excess of O2, diffusion of hydrogen into the bulk is blocked by adsorbed oxygen and the hydrogen reservoir available for reaction at the surface is decreased by several orders of magnitude. This results in a drastic reduction of the reaction rate which can be reversed by increasing the partial pressure of H2. © 1979.