Employing photoelectron emission microscopy (PEEM) as the spatially resolving technique, pattern formation during the catalytic redn. 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 lithog. 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.