Nanostructured Sn-doped TiO2 have been prepared by ball milling using SnO2 and TiO2 as raw materials. The as-prepared powders are characterized by XRD, SEMs and EDAX to identify the structural phases, surface morphology, and composition of the materials. The materials are prepared with the addition of tin of different molar ratios (0, 0.05, 0.10, 0.15, 0.20, 0.25, and 1.0) to TiO2 and sintered at 800°C for 3？h. They are subjected to dc resistance measurements as a function of relative humidity (RH) in the range of 30%–97% in a self-designed humidity chamber, and the results revealed that the sensitivity factor increased with an increase in tin molar ratio. Among them, TiO2—20？wt% of SnO2 possessed the highest humidity sensitivity, while the pure TiO2 and SnO2 composite possessed a low sensitivity. 1. Introduction The development of humidity sensors has received much attention during the last years due to the necessity of controlling and monitoring environments in many different fields like industrial processes and domestic comfort [1–3]. Since each application field requests distinct operating conditions and generally sensor elements work in narrow ranges of humidity and temperature, the selection of a material should be based on certain conditions, in order to assure the satisfactory operation of the humidity sensor, that include good sensitivity, linearity over the range of application, fast response, low hysteresis, and stability in the exhibition to impurities present in the environment [3–6]. As a consequence, a wide variety of materials have been researched with the objective to study their sensitivity, usually variations of electrical parameters in relation to the humidity in the atmosphere. These materials are based on polymers, electrolytes, and especially ceramics [1, 3]. At present, ceramic materials possess a certain prominence due to their properties, which exhibit advantages regarding their mechanical resistance, and physical and chemical stability. The humidity-sensitivity properties of a ceramic are mostly influenced through the porous microstructure and the surface reactivity with moisture. Porous ceramic materials based on metals oxides have been largely used as humidity sensors [2]. The principle of humidity measurement with ceramic sensors is the change in electrical capacitance and conductance owing to water vapor chemisorption and physisorption and/or capillary condensation in the pores of the ceramic sensor. TiO2-based humidity sensors are studied vigorously, as it is likely to show better sensitivity because of its hydrophilic
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