A microcalorimetric method has been developed which enables the rates, orders and apparent activation energies of a heterogeneous catalytic reaction to be rapidly measured. The method has been used to study the complete oxidation of methane on a series of palladium-gold alloys. The results indicate the participation of the d orbitals of the catalyst in the reaction. Evidence has also been found for the existence of a relatively stable adsorbed intermediate in the reaction sequence. The changes in apparent activation energies are attributed, at least in part, to changes in the heat of formation of this intermediate.