Relatively small amounts of H-2 (500-3000 ppm) have been added to a real diesel exhaust gas during passive hydrocarbon-selective catalytic reduction (SCR) over a silver catalyst. The added H-2 is very effective at promoting low-temperature activity and at sustaining it by removing carbon-rich poisoning species from the catalyst surface. NO2 production by the catalyst appears to be a key step in the self-cleaning process, but it is highly dependent upon the gas-phase concentrations of H-2, total NOx, and hydrocarbons and the exhaust-gas temperature. Careful control of H-2 addition is therefore required in response to changes in conditions, to maintain suppression of the carbon-rich species without releasing NO2 from the tailpipe. The highest overall NOx reduction can be achieved by operating the engine with exhaust gas recirculation in combination with H-2-promoted passive hydrocarbon-SCR as an aftertreatment.