The selective catalytic reduction (SCR) of NOx in oxygen excess is reported to proceed with good efficiency by NH4+ ions fixed to zeolites at temperatures as low as 373 K. The reaction starts with almost complete reduction of NOx to N-2 under consumption of NH4+ ions thereby converting the zeolite into its H+ form. The consumed NH4+ ions can easily be replenished at reaction temperature by admission of gaseous ammonia. A key step of the reaction sequence is the oxidation of NO to NO2 by O-2 which is catalyzed by the zeolite. Both properties of the zeolite, namely, its storage capacity for NH3 (in the form of NH4+ ions) and its oxidation capability for NO allow the abatement of NOx in exhaust gases at low temperatures and in the presence of oxygen without permanent addition of a reductant. This process is named catalytic low temperature conversion (CLTC) of NOx.