TiO2-based ceramic humidity sensors containing small amounts of Li2O and V2O5 were prepared by a solid-reaction method. The relationship between the pore structures of the sensors and one of the most important humidity properties, hysteresis in humidity sensitivity, was studied. The humidity sensitivity was investigated by measuring the impedance of the sensors under various conditions of humidity, and the pore-size distribution was measured by mercury-intrusion porosimetry and by the nitrogen-adsorption method. The hysteresis property of the sensors was shown to be significantly dependent on their microstructure, particularly the volume ratio of mesopores to all pores in the sensor elements. Nitrogen adsorption was found to be more useful than mercury porosimetry for determining the proportion of mesopores in the samples.