The oxidation of methane on alumina-supported platinum catalysts has been investigated for various gas mixtures as a function of the state of oxidation or reduction of the platinum surface. It has been shown that the activity for methane oxidation goes through a sharp maximum a few seconds after the introduction of the reactants to either a pre-reduced or a pre-oxidised catalyst. The results are interpreted in terms of a Langmuir-Hinshelwood mechanism in which the optimum rate of reaction corresponds to a platinum surface partially covered by both oxygen- and carbon-containing species. The position of the maximum in activity is very reproducible for pre-reduced catalysts but varies in an imprecise manner for pre-oxidised catalysts. These effects are thought to be related to morphological changes in the reduced and subsequently re-oxidised small platinum particles. It appears that even after re-oxidation at 300 degrees C these platinum particles have a strong ’memory’ of previous reduction treatments.