Recently the rheotaxial growth and thermal oxidation (RGTO) technique has been proposed as a new method for preparing gas-sensitive SnO2 films with a high surface-to-volume ratio. Such films are produced in two stages : in the first step small metallic Sn droplets are formed by evaporating or sputtering Sn onto substrates heated beyond the Sn melting point (T-s > T-m = 232 degrees C) and, in a second step, these spheres are oxidized by thermal annealing in air (T-ox > 500 degrees C). In this situation, the oxidation kinetics is limited by the diffusion of oxygen through the growing surface SnO2 layer towards the metallic grain interior in incompletely oxidized RGTO-SnO2 films. In the present work, the oxidation kinetics of the Sn spheres has been studied using optical absorption measurements. From our measurements we obtain for the temperature dependence of the oxygen diffusion coefficient in tin oxide D(T)= 1.4 x 10(-4) cm s(-1) exp(-1.67 eV (kT)(-1)). The consequences of this result are discussed with regard to the preparation of optimized RGTO-SnO2 films and to sensor drift phenomena.