Catalysts based on rhodium supported on zirconia show a remarkable in situ activation in N2O decomposition after treatment at 500 degrees C in a flow of 0.05% N2O in helium. The effect, specific for the presence of zirconia as a support, is observed even in the presence of an oxygen concentration two orders of magnitude higher than that of N2O, but disappears when water in combination with oxygen is present in the feed. The effect is interpreted as being due to the stabilisation, on the dehydroxylated zirconia surface, of oxygen vacancies at the interface between the support and Rh particles. These vacancies help the migration of the oxygen released by N2O decomposition on the Rh surface to zirconia and then its desorption as O-2. On a hydroxylated zirconia surface, this process is inhibited and the Rh surface becomes covered by the oxygen atoms produced by N2O dissociation, with a lowering of its intrinsic activity in N2O decomposition.