Water isotope exchange in the presence of CO on two undoped tin dioxides has been studied using modulation excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and resistance measurements at 300 ºC. Our results reveal that the material synthesized from tin tetrachloride (SnO₂Cl) manifests higher affinity to chemisorbed water than that made from tin hydroxide acetate (SnO₂Ac). The latter was shown to exhibit a strong correlation between the evolution of surface OH groups (bridging type, involved in hydrogen bonding) and electric resistance upon increasing concentration of CO. Water desorption kinetics, being independent of CO concentration for both materials, was found to be slower for SnO₂Cl by ca. 30% with respect to SnO₂Ac. High affinity to water as well as low sensor signals to CO in humid air reported for SnO₂Cl were proposed to originate from traces of Cl ions (about 0.15 wt % for SnO₂Cl and 0.03 wt % for SnO₂Ac) and not microstructure, which has been confirmed to be similar for both materials. Two types of water adsorption and two CO sensing mechanisms are proposed for SnO₂Cl and SnO₂Ac on the basis of the results.