In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH3 only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst : calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings.