The reduction of NO2 to N-2 at temperatures as low as 150 degrees C without the use of an externally supplied reductant can be achieved with a microporous solid, (NH4)(3)PW12O40. Temperature-programmed experiments with NO, NO2 and ammonium salts labelled with N-15 show that NH3 is not released from the solid until a temperature of 400 degrees C or greater is attained. These observations, together with the high selectivities to (NN)-N-15, show that NOx is predominantly interacting with the bound NH3 rather than that in the gas phase. After depletion of the NH3 bound as NH4+ the parent acid remains, on which additional NOx is sorbed irreversibly. The sorbed NOx can be removed from the solid by water treatment at elevated temperatures and the ammonium salt can be regenerated by a precipitation process or by direct injection of NH3 into the catalyst bed when the process is off-line.