Sodium strongly promotes the reduction of NO by propene over Pd catalysts supported on 8 mol% yttria stabilised zirconia (YSZ) in the temperature range 520-720 K. Rate increases by an order of magnitude are achievable, while the selectivity towards to N-2 is improved from similar to 75% over the unpromoted Pd catalyst to >95% over the optimally Na-promoted catalyst. The results indicate that promotion is due to sodium-induced increases in the strength of NO chemisorption relative to propene. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which process is proposed as the critical reaction initiating step. The XPS and Auger data are consistent with this view. These findings are in excellent accord with a recent study of the same reaction under conditions of electrochemical promotion (EP) (I. V. Yentekakis et al., J. PhJ s. Chem. B 101, 3759 (1997)).