The effect of steam on NO x reduction over lean NO x trap (LNT) Pt–Ba/Al2O3 and Pt/Al2O3 model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C3H8 as reductants, respectively. Compared to dry atmosphere, steam promoted NO x reduction; however, under rich conditions the primary reduction product was NH3. The results of NO x reduction and NH3 selectivity versus temperature, combined with temperature programmed reduction of stored NO x over Pt–BaO/Al2O3 suggest that steam causes NH3 formation over Pt sites via reduction of NO x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C3H8/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO x contributed to NH3 formation. The NH3 formed under these conditions could be effectively trapped by a downstream bed of Co2+ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO x reduction, leading to improved NO x storage efficiency.