The reversible condensation of potassium into large K + O coadsorption islands that occurs during the O-2 + H-2 reaction on a Rh(110) surface predosed with potassium has been studied in the 10(-7) mbar range at temperatures between 533 and 623 K and with K coverages between 0.025 and 0.11 ML. Scanning photoelectron microscopy has been used as a spatially resolving in situ method to follow the development of the condensation process via calibrated K and O profiles. A Pt patch evaporated onto the Rh(110) surface served as a nucleation center for the reaction fronts. The coadsorbate phases evolving in the course of the condensation were characterized by local photoelectron spectra. Only at high enough temperature (similar to580 K) is a nearly complete redistribution of the potassium by reaction fronts observed, whereas at low temperature, a substantial amount of the K + O coadsorbate remains on the reduced surface. The formation of a high-coverage (8 x 2) K + O phase characterized by an energy-shifted component in the Rh 3d and O 1s peaks appears to play a particular role in the condensation process.