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A method which can be used for t h e systematic design of gas-sensitive resistors is presented for quantitative evaluation of t h e kinetic parameters for t h e catalysed decomposition of certain gases on t h e surface of semiconducting oxides. The method is based on the measurement of the response to the presence of the gas of the electrical resistivity of t h e material, measured at different spatially separated regions. The analysis is presented for a disc geometry, with electrodes at t h e centre and periphery of t h e disc. For two different oxides, SnO, and Ba,Fe, ,5Nb8.5030 , a transition with increasing gas concentration from first-order to zeroth-order kinetics for combustion of propane and carbon monoxide is shown and t h e results are related to t h e variation of t h e surface coverage of adsorbed oxygen species. Rate constants for oxygen adsorption are derived. The surface oxygen species which controls t h e combustion rate is not t h e s a m e as that which mediates t h e conductivity change.

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