The influence of some parameters on the electrical conductivity of polycristalline stanic dioxide has been studied in use of gas detection. A first various samples were obtained by changing sintering parameters (temperature, atmosphere...) they have been caracterized by scanning electron microscopy (grain size), measure of thr specific surface area and the porosity. Then the influence of various grain sizes on the electrical conductivity is studied. These measures were done under different hot gaseous atmospheres. The conditions of the preparation of the samples influence the selectivity and more especially the stability of such detectors. Above all the control of grain boundaries in such material is important.