Abstract A tin oxide semiconductor modified with CaO is found to be especially sensitive to odor molecules and to have some sensing characteristics perfectly suited to the detection of odor. The sensitivity to more than 70 species of organic or inorganic molecules associated with odor has been measured in the present study. The sensitivity increased with increase in the carbon number, the degree of unsaturation, the electronegativity and the number of methyl groups. The sensitivity-concentration relationship is a power law in the ppm concentration region. The sensing characteristics are discussed in connection with some physicochemical properties of the odor molecules and the odor intensity-concentration relationship.