The spatial distribution of potassium on a Rh(1 1 0) surface during the catalytic O-2 + H-2 reaction is investigated employing scanning photoelectron microscopy (SPEM) and photoelectron emission microscopy (PEEM) as spatially resolving in situ methods. Mass transport of potassium with the reaction fronts is observed leading to a slowing down of the fronts and to time-dependent concentration profiles. Differences in the mobility and in the bonding strength of potassium on the 'reduced' and on the oxygen covered surface are shown to be the key factors for the enrichment of potassium in the front region. (C) 2002 Elsevier Science B.V. All rights reserved.