Multiplicity was experimentally observed during steady-state and periodic lean NOX reduction with propylene on a model Pt/BaO/alumina monolithic catalyst. The dependencies of the monolith temperature, hydrocarbon and NOX conversion were determined as a function of feed temperature in both steady-state and periodic operation. The nonisothermal multiplicity is attributed to the exothermic propylene oxidation and thermal feedback in the solid phase. The steady-state NOX conversion achieves a maximum within the region of multiplicity and very near the extinction point. Periodic experiments, which involve switching between lean and rich operation, reveal an expanding region of multiple states as the amount of propylene is increased by increasing the rich pulse duration. A NOX trap reactor model confirms the existence of multiple branches of states during both steady-state and periodic operation, and captures the main trends that are experimentally observed. (c) 2007 Elsevier Ltd. All rights reserved.