Tin oxide samples are prepared from a powder under various sintering conditions. The morphological properties are studied. The electrical properties are correlated to the structural and textural characteristics, and specially in relation with the grain size. The influence of the gaseous atmosphere (humidity and various gases) on the electrical conductivity is investigated. Some catalytic experiments are performed in order to study the role of the analysed gas, the influence of the sintering conditions and the nature of the electrodes. All these factors modify the electrical conductance. The conductivity power law dependences are carried out in order to study the defect chemistry in tin oxide. The results are interpreted in relation to a theoretical model.