The O-2 + H-2 reaction has been studied under low pressure conditions (10(-5) mbar) employing a microstructured Rh(110)/Pt surface as catalyst. Photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM) were used as spatially resolving in situ methods. Under reaction conditions stationary concentration patterns (Turing-like structures) of the adsorbates develop inside the Pt domains which are associated with a compositional change of the metallic substrate.