A sol-gel process, followed by supercritical fluid drying (SCFD) was employed to prepare ultrafine tin oxide. Two different procedures were introduced and the powder properties of these two products were measured and compared, including the particle size, specific surface area and pore size distribution. Because the same reactive mechanism occurs in the gas sensing process and catalytic process, the sensor properties can be improved largely by the use of ultrafine tin oxide. In this paper, different gas sensing properties were observed when the two products were used as base materials of gas sensors. The effects of pH value on the pore structure and response (reversion) time of the sensors are also discussed.