The effects of surface modifications of SnO2-based CO gas sensors with iridium and/or ruthenium on the temperature and humidity dependence were investigated. The surface modification was carried out by dipping the sensor element in a dilute metal salt solution for only a few seconds, then heating it at 600 degrees C for 5 min. Differences in sensor resistance in 100 ppm CO at varied temperature and humidity conditions were compared between unmodified and modified sensors. The results indicated that the surface modifications with the platinum group metals effectively improved the temperature and humidity dependence; a decrease in sensor resistance with lowering temperature and humidity was suppressed by the surface modifications with iridium or ruthenium or a combination of both metals. Surface modification using sulfuric acid and thiourea solutions, which was shown in our previous work to enhance long-term stability of the sensor, was employed prior to the modification with the platinum group metals. It was demonstrated that this mixed surface modification resulted in further improvement of temperature and humidity dependence, and induced excellent long-term stability of the SnO2-based CO gas sensor.