Prototype NO, sensors for use in automotive exhaust can contain Au/Pt electrodes that dissociate O-2 for removal by electrochemical pumping, but these electrodes should not dissociate NO. In these sensors, the NO diffuses to a second chamber where it is then detected. To understand the process that occurs at the Au/Pt electrode we have used temperature-programmed desorption to study the dissociation of NO and O-2 On Pt(335) and on partially Au covered Pt(335). With a gold coverage of 0.15 ML, the dissociation probability of NO is decreased by a factor of 5 relative to bare Pt. Increasing Au coverage to 0.3 ML decreases NO dissociation to an undetectable level. At 0.3 ML Au, the saturation coverage of atomic O is reduced by only 20%. The variation of NO dissociation with Au coverage occurs in the range where all of the Au atoms are at steps, but Au is more effective at decreasing NO dissociation than can be explained by a simple site-blocking model. The effect of adsorbed O on NO dissociation was also investigated. Both Au and O decrease NO dissociation, but An is much more effective. Blocking 40% of the step sites with O reduces NO dissociation by 70%-more than site-blocking, but much less than a comparable coverage of Au. Our observations show how the Au/Pt electrode in a NOx sensor is able to dissociate oxygen similar to 10(3) times more efficiently than MO.