The heterogeneous reaction of NO, SO2 and O2 in the presence of condensed or adsorbed H2O results in high yields of N2O under conditions mimicking flue gas from fossil fuel combustion. We observed a strong influence of the nature of the substrate surface on the rate of N2O formation. The maximum rate of N2O formation of 350 ppm h-1 occurred on soot and fly ash at relative humidities approaching 100% at 368 K final yields of 80--100% N2O. The mechanism corresponds to a complex multiphase system in which NO2 seems to be a key species. The observed abundance of NO2 does not correspond to the one predicted by the simple mechanism by Lyon and Cole (1988). ¿ American Geophysical Union 1995