The formation of SnO2 films for gas-sensing applications through the thermal oxidation of tin is analyzed in detail. Special emphasis is given to the influence of an incomplete oxidation on the stability of the sensor and its evolution as source of drift of the sensor response. For this, a complete structural characterization of the thermal oxidation of tin droplets is presented. Intermediate phases appearing during the transformation from Sn to SnO2 are identified and discussed. Likewise, the origin and changes of the spongy agglomerates appearing during oxidation are discussed. Comparison of the behavior of sensors obtained at different stages of the oxidation process shows the effect of the completion of the oxidation on the stability of the sensor response.