Modulation excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) together with resistance
measurements has been carried out to study water isotopic exchange on undoped SnO<sub>2</sub> materials as a function of CO concentration. We compare two materials synthesized via hydrothermal treatment and different only in their precursors: SnO<sub>2</sub> Ac synthesized from tin(IV) hydroxide acetate and SnO<sub>2</sub> Cl from tin(IV) chloride pentahydrate. DRIFTS and resistance measurements were performed simultaneously in an environmental chamber at 300 oC and in a flow of humid air. The annealed materials were found to have similar particle sizes (16±7 nm), crystallite sizes (12±2 nm) and pore size distribution (9±1 nm). However, sensor tests showed notably higher responses to CO in the presence of water vapor for
SnO<sub>2</sub> Ac. Electronic effect of CO chemisorption quantitatively correlates with consumption of bridging hydroxyls on the latter surface upon increasing concentration of CO from 0 to 500 ppm in humid air. No such correlation was found for SnO<sub>2</sub> Cl. Water desorption kinetics was found to be slower for the latter by ca. 30 % with respect to SnO<sub>2</sub> Ac. Low
activity of surface OH groups and consequently low sensor signals of SnO<sub>2</sub> Cl were proposed to originate from traces of Cl ions found in the material after the synthesis despite negative Cl test before the hydrothermal treatment.