Cadmium sulphide filins have been deposited on glass substrate and the effect of heat treatment on the electrical transport is studied. The current increases with the time of heating and shows a saturation tendency after a certain time of heating. The above features may be attributed to oxygen diffusion into the CdS film. The average density and diameter of the grains have been determined from SEM analysis. The observed change in the electrical characteristics upon heat treatment has been correlated to oxygen from a theoretical analysis based on modified thermionic emission model applicable to polycrystalline semiconductor.