Under oxygen-deficient (rich) conditions, a potential route to maintaining control of carbon monoxide (CO) and hydrocarbon (HC) emissions in a three-way system is through reactions of these pollutants with water vapor (H2O). The importance of such reactions over supported rhodium was investigated in the laboratory. The water-gas shift (CO + H2O) was insignificant. Steam reforming (HC + H2O) took place, but gave CO as a product. Thus, if CO conversion governs the rich-side effectiveness of a three-way catalyst, steam reforming which converts HC to CO is unlikely, in itself, to improve three-way performance.