The effects of post-deposition heat treatment on some polycrystalline samples of SnO2 thin films prepared by the pyrosol process have been reported. It is observed that heat treatment in different ambients have remarkable effects on the film resistivity. No appreciable change of carrier concentration and optical transmission have been noticed, therefore the change of resistivity is considered to be due to the change of mobility of the carriers. The Hall mobility in SnO2 thin films, having carrier concentration >or approximately=1019 cm-3, is thermally activated in the investigated temperature range of 27 to 250 degrees C. The oxygen diffusion mechanism at the grain boundary of the films has been discussed critically to explain the observed mobility data. It is concluded that the role of the oxygen diffusion process in the film is only to modulate the inter-grain boundary potential energy barrier height.