A novel method to modify the surfaces of SnO₂-based CO gas sensors in order to improve their temperature and humidity dependence has been developed. This surface modification is accomplished by dipping the sensor element in a dilute aqueous solution containing platinum and/or iridium for only a few seconds, then heating it at 600℃ for 5 min. This simple method effectively suppresses variations in sensor resistance to temperature and humidity in a specific concentration of CO; a decrease in sensor resistance with lowering temperature and humidity is prevented by the surface modifications with platinum and/or iridium. This effect is more enhanced by using both platinum and iridium than by using either metal individually. Moreover, preliminary surface treatments with sulfuric acid and thiourea further reduce the temperature and humidity dependence of sensor resistance in CO. It has been also proven that the modification of the SnO₂-based CO gas sensor's surface using mixed elements results in its excellent long-term stability.