A process for reducing the nitrogen oxides content of the exhaust gas from an internal combustion engine includes treating the exhaust gas in an electric gas discharge such that at least one of nitrogen dioxide or nitric acid are formed from nitrogen oxides in the exhaust gas; storing the nitrogen dioxides and/or the nitric acid in the form of nitrates on a nitrogen oxides storage material during phases when the engine operates with a greater than a stoichiometric air/fuel ratio (i.e., the storage phase); decomposing the nitrates to give nitrogen oxides during brief phases when the engine operates with a less than the stoichiometric air/fuel ratio (i.e., the desorption phase); and reducing the nitrogen oxides which are released from the storage material to nitrogen. The nitrogen oxides storage material may be located in an electric gas discharge zone, and the exhaust gas may be passed through the discharge and then over the storage material.