Oxygen adsorption plays key roles in resistive-type SnO 2 gas sensors that can very sensitively detect combustible gases such as CO and hydrocarbons. Thus, exact evaluation of the electric resistance of SnO 2 in response to oxygen is important to understand the oxygen adsorption mechanism. However, infinitesimal impurities contained in even high-grade commercial oxygen cylinders impose great effect on the sensitivity of SnO 2. In this study, we designed an experimental system, which composed of gas pretreatment chambers including a Pt/Al 2 O 3 combustion catalyst and a zeolite adsorbent, for exact analysis of gas sensing properties. Our experimental system allowed for the accurate determination of the dependence of the electrical resistance (R) on oxygen partial pressure (P O2) by removing impurities in sample gases. According to the linear correlation between R versus P O2 1/4, we concluded that oxygen adsorbed on the SnO 2 surface in the form of O 2− at 350 and 450 C in extremely dry conditions that was achieved using the experimental system. The competitive adsorption on the SnO 2 surface in the form of O− and O 2− was suggested at 300 C.