Partial retention of chloride by Pt/gamma-alumina catalysts prepared from chlorine-containing precursors markedly poisons both activity and nitrogen selectivity in the reduction of NO by propene. In the complete absence of chloride, Ba acts as a very strong promoter, dramatically improving activity, nitrogen selectivity and NO conversion. Very strikingly, when Ba is used to promote a catalyst prepared from chloride-containing precursors, all the original chloride is retained, yet the catalyst exhibits very strong promotion, and in every aspect its behavior is identical to that of the chloride-free Ba-promoted catalyst. This behavior is rationalized in terms of BaCl2 formation and the relative effects of Cl and Ba in inhibiting and promoting, respectively, the dissociation of chemisorbed NO-the reaction-initiating step.