The formation of stationary concentration patterns that develop in the O-2+H-2 reaction on a microstructured Rh(110)/Pt surface predosed with potassium (theta(K)approximate to0.08) has been investigated with photoelectron emission microscopy and scanning photoelectron microscopy. The reaction was studied in the 10(-7) and 10(-6) mbar range between Tapproximate to500-600 K on a Rh(110) surface onto which a square-shaped Pt domain of 70x70 mum(2) size and 70 Angstrom thickness had been deposited in UHV. Two types of Pt domains, one with pure Pt and one with Rh alloying, were prepared. The chemical state of the metallic substrate and of the adsorbate layers were characterized in situ by local (spot size 0.15 mum) photoelectron spectra of the Pt 4f, Rh 3d, K 2p and O 1s levels. Stationary concentration patterns consisting of K+O condensation islands of macroscopic size developed on both types of Pt domains under reaction conditions. On the alloyed Pt patch the formation of the large K+O condensation islands was associated with the reversible local segregation of Rh. The K+O condensation is interpreted as primary pattern forming process, while the segregation of Rh is a secondary process, which follows the distribution of oxygen resulting from the primary process.