In this study, the influence of the oxygen partial pressures (PO2) on the sensor response to H-2 of SnO2 resistive-type gas sensors was evaluated under various humid atmospheres. SnO2 nanoparticles of 8-15 nm in diameter were synthesized using a hydrothermal technique followed by calcination at 600 degrees C. Additionally, a large amount of pores with diameters greater than 10 nm was confirmed in the nanoparticles. The electrical resistance at 350 degrees C was decreased with decreasing the P-O2, and the electrical resistance in the presence of 10 ppm H-2 was much smaller than that in the absence of H-2 in both dry and humid atmospheres regardless of the P-O2. Furthermore, the sensor response to 10 ppm H-2 at 350 degrees C increased with decreasing P-O2 in both dry and humid atmospheres. Thus, decreasing the amount of oxygen adsorption enhanced the effect of rooted hydroxyl formation on the SnO2 surface through a combustion reaction between H-2 and adsorbed oxygen and improved the sensor response to H-2. These results are important for understanding the fundamental mechanisms of gas detection and for the material surface design of highly sensitive resistive-type semiconductor gas sensors. (C) 2019 The Electrochemical Society.