A highly reactive reducing gas mixture is produced from vehicle fuel and introduced into the exhaust gas of an internal combustion engine operated at lean burn conditions and passed over a reducing catalyst to convert NO.sub.X emissions to benign emissions. Preferably, fuel with oxygen present in a carrier gas is metered into a plasma reactor having a bed of dielectric particles which prevent formation of coke as the fuel is reacted. The plasma induces a number of simultaneous reactions with the fuel to produce a substantial amount of oxygenated and non-oxygenated, unbranched organic molecules at relatively low temperatures that are highly reactive and ideally suited for use as a reducing agent in an SCR catalyst.