The variation of surface conductivity of SnO2, Sb2O3 and of antimony-doped tin dioxide in air, argon, propylene-air, and propylene-argon mixtures has been followed in situ between room temperature and 400 degrees C. The apparent p-type behavior of some of the antimony-doped samples was explained by fluctuations in the concentration of the surface defects caused by variation of the ratio of coexisting Sb3+ and Sb5+ ions. Thermogravimetric data indicate increased lattice mobility on antimony doping. It is suggested that the high electronic conductivity resulting from the presence of antimony in the SnO2 matrix is related with the increased structural disorder; consequently, the increased rate of the reoxidation step prevents surface contamination with strongly adsorbed hydrocarbon species.