The effect of preadsorbed potassium on the pattern forming NO + H-2 reaction on Rh(110) has been studied in the 10(-7) mbar range using photoelectron emission microscopy (PEEM) and scanning photoelectron microscopy (SPEM) as spatially resolving methods. Domain patterns of travelling wave fragments develop with pulses in adjacent domains moving in opposite directions parallel to the [1 (1) over bar0] direction. 2D-elemental maps from in situ SPEM measurements show that potassium is concentrated in the oxygen covered part of the pattern whereas the nitrogen covered parts are largely free of potassium. (C) 2004 Elsevier B.V. All rights reserved.