The nature of the active sites, the role of the support, and the mechanism by which hydrocarbons:are activated over supported Pt catalysts have been investigated for the combustion of propane in the presence and absence of SO2. A strong enhancement in the activity for propane oxidation has been confirmed either when SO2 is introduced with the propane or with a pre-sulphated alumina-supported catalyst. No equivalent effects were found with silica-supported catalysts. Fluorination of the alumina support also leads to an increase in activity. The addition of pulses of SO2 into the propane-containing gas stream produces a very large, but short-lived, increase in activity in addition to a more gradual and progressive activity enhancement. Reasons for these different effects are discussed. Attempts to correlate the permanent enhancement in activity with the total acidity of the support were unsuccessful. It appears that the increase in activity is due to a more subtle effect and a model is presented in which the possible role of perimeter sites at the metal-support interface is emphasised.