A Na+ ion conducting ZSM-5 zeolite is applied as the sensitive phase in a potentiometric gas sensor consisting of the cell Au/(Na2CO3,BaCO3)/zeolite/Au, where the Na2CO3/Au system serves as the reference electrode. The cell voltage shows a linear response to changes of the logarithm of propane partial pressure (200-4000 Pa) in CO2/O-2/N-2 at 673 K. The proposed mechanism assumes Na or Na+ activity changes in the zeolite pores due to hydrocarbon sorption. An independent effect is a weak response of the zeolite/Au electrode to O-2 and CO2 due to oxide and carbonate formation. (c) 2006 The Electrochemical Society.