The bonding character of oxygen and potassium and the interactions in K+O coadsorbed layers on a Rh(110) surface have been studied by means of high resolution x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The Rh 3d(5/2), K 2p, and O 1s spectra and LEED patterns were used as fingerprints for the interfacial reactions and the structural changes. Dramatic changes in the chemical state of the substrate occur in the presence of dense K+O adlayers, when the oxygen coverage exceeds one monolayer. The effect of coadsorbed potassium on the "reactivity" of oxygen was probed by following the evolution of the O 1s spectra during titration with H-2. The enhanced surface capacity for oxygen adsorption and the reduced rate of H2O formation with increasing K coverage were discussed considering the influence of K on the dissociative adsorption of O-2 and H-2 and on the bonding of the coadsorbed species. (C) 2003 American Institute of Physics.