An assembly and method for gas analysis. The assembly comprises a catalyst compartment for catalytically reacting a component of a gas sample, producing one or more gas species as products. A product compartment receives the gas species, and a sensing element within the compartment senses the amount of one or more of the gas species. This amount is compared to the amount of the same gas species present in a reference compartment containing a non-catalyzed gas sample, providing the amount of the gas species produced by catalysis. Using this value, the content of the gas component in the gas sample is calculated based upon the stoichiometry of the catalyzed reaction. In preferred embodiments, the gas for analysis is a process gas for fuel production, and the catalyst is a high temperature shift catalyst that catalyzes the reaction of carbon monoxide and water into hydrogen and carbon dioxide.