The reduction of NOx under lean-burn conditions remains a challenging technical and scientific problem. In this paper, a number of catalytic systems are investigated by steady state testing and temperature-programmed desorption/reaction and the results discussed in terms of the similarities between the reactions in an attempt to rationalise apparently unrelated results. The reaction mechanisms are divided into two classes. (1) Reactions where NOx reduction occurs on the Pt surface, (e.g. C3H6NO-O-2 reaction over Pt/Al2O3) which are active at the lowest temperatures and are resistant to sulphur poisoning. (2) DeNO(x) reactions on Al2O3 With a weakly adsorbed reductant, (e.g. C3H8-NO-O-2 reaction over Pt/Al2O3 and Al2O3 and the C3H8NO2-O-2 reaction over Al2O3) which are strongly poisoned by sulphur and appear to occur via the formation of a surface nitrate species on the Al2O3 Which activates the reductant.