Thin SnO2 films of about 10 nm thickness were prepared by sputtering on SiO2/Si substrates, and the effects of an external electric field from the substrate on the conductance of the SnO2 film were investigated at 26 degrees C and up to 300 degrees C. The apparent conductance of the SnO2 film increased with a positive substrate bias and decreased with a negative substrate bias. These field effects became more evident with larger bias and in SnO2 films with a lower carrier concentration. By using metal oxide semiconductor theory, the results are attributed to the formation of a depletion and an accumulation layers in the SnO, film under a negative and a positive bias, respectively. The influence of electron traps in the SnO2 film on its apparent conductance was suggested from calculations of these field effects. At temperatures above 150 degrees C, slow changes in the conductance with time were observed under a constant negative bias. From the results measured in N-2 and in air, it was suggested that the amount or types of surface oxygen adsorbates changed under a negative electric field. These results indicate the possibility of controlling the electrical properties and the surface reactions of SnO2 films by an external electric field.