One of the main problems of undoped SnO2 based sensors in real life applications is their lack of selectivity and the most encountered cross interference comes from the water vapor. Its effect is twofold: it decreases the sensor resistance and its presence in various concentration changes the response magnitude under exposure to identical target gas concentrations. It is also known that the addition of small quantities of noble metals has a considerable impact on the gas sensing with SnO2 based sensors; among others, it also influences the effect of water vapor on sensing. The impact of the presence of different concentrations of water vapor on the CO sensing is very different for the undoped and doped materials. For the undoped one, the sensor signal is extremely decreased, whereas the doped materials exhibit higher sensor responses in the presence of humidity. In order to understand the –obviously - different reaction mechanism on the different gas sensors, measurements with different test gases (H2O, D2O, H2, D2, CO) and in different atmospheres (N2 and air) were performed. The application of an operando set up (combined measurements in real life conditions) enabled the combination of phenomenological (DC resistance) and spectroscopic measurement techniques (DRIFTS) and the correlation of electrical effects with changes on the surface of the working sensor. In this way it was possible to identify the reaction partners of the test gases on the different surfaces and explain the – one enhancing once diminishing – interfering effect of water vapor on the CO sensing.