Pattern formation in the NO + H 2 reaction on Rh(111) has been investigated in the 10 - 6 mbar range using photoelectron emission microscopy (PEEM) as a spatially resolving method. Target patterns, spiral waves and irregular patterns are observed in a T-window of ∼ 20 K width at around 460 K, located in the transition range between the reactive and unreactive states of the surface. A new species characterized by a work function below that of the clean surface forms upon collision of two wave fronts. This new species is tentatively assigned to subsurface oxygen. The colliding wave fronts start to interact when they are still more than 100 μm apart, thus demonstrating the presence of a long-range diffusional coupling.