Ambient temperature SCR of NO was studied over Ti, V and B silicalite-1, silicalite-1 and H-ZSM-5 catalysts. It was observed that even at 0 degrees C silicalite-l possesses some activity which falls off sharply with increasing temperature. An enhanced activity could be correlated to the titanium content in the Ti and Ti-V silicalite samples. H-ZSM-5 and H-BS-1 show an unusually high transient activity probably due to weakly adsorbed ammonia species. These would not be adsorbed at higher temperature thus explaining the decreased activity as temperature is raised. Indeed with these catalysts no transient high activity was observed when the reaction was carried out at decreasing temperatures.