The NO + H 2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatiotemporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10 À7 and 10 À6 mbar pressure range, with the potassium coverage varying between y K = 0.05 and y K = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation:coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium.