Employing photoelectron emission microscopy (PEEM) as the spatially resolving technique, pattern formation during the catalytic reduction of NO with CO has been investigated on a microstructured Pt(100)/Ti/TiO2 surface in the 10(-6) and 10(-5) mbar range. The microstructured surface-initially created by a lithographic technique-shows restricted Pt(100) domains of varying size and geometry, such as circles, rings, and dumbbells, surrounded by an inert Ti/TiO2 layer. It is shown that pattern formation during the NO+CO reaction, i.e., the propagation of pulses on the Pt(100) surface, is significantly affected by the size and the geometry of these domains. (C) 2000 American Institute of Physics. [S0021- 9606(00)70715-X].