Ba(Ce,Pd)O 3 perovskite is an unusual catalyst for CO oxidation by O 2 , given its low surface area. The rate law for the catalytic reaction was evaluated under CO-lean and CO-rich conditions, as well as near-stoichiometric conditions. When O 2 is present in excess, the kinetics show CO inhibition, consistent with a Langmuir–Hinshelwood mechanism in which both reactants compete for the same adsorption sites. The Arrhenius activation energy for this mechanism is surprisingly low, (7.8±0.3)kcal/mol. It is attributed to weak adsorption of CO on ionic surface Pd(II) sites. When O 2 is limiting, the reaction orders for both CO and O 2 show a strong dependence on P(CO)/P(O 2 ), and eventually become independent of both P(CO) and P(O 2 ) at high P(CO). This suggests a new BaCeO 3 -mediated mechanism which dominates the reaction at high P(CO)/P(O 2 ). Its Arrhenius activation energy is (9.7±0.3)kcal/mol, probably reflecting the barrier to oxygen migration in the oxide phase. Both mechanisms contribute significantly for P(CO)/P(O 2 )≈1. The catalytic activity of Pd-substituted BaCeO 3 is attributed to the increased bulk oxygen mobility in the presence of square-planar Pd(II) ions that are located on the perovskite B-sites, each adjacent to an oxygen vacancy.