Techniques of thermoelectric and photothermoelectric analysis have been applied to investigate the electrical transport properties of both solution‐sprayed and evaporated polycrystalline CdS films. Dark electron densities are independent of temperature between 200 and 330°K because of shallow donors; dark electron mobilities are thermally activated with energies between 0.09 and 0.23 eV. Photoconductivity of solution‐sprayed films is caused primarily by an increase in electron mobility. Other effects investigated are the adsorption and desorption of oxygen, the diffusion of Cu acceptors, and field‐assisted drift of Cu impurities in the film at a slightly elevated temperature (100°C). The structure and characteristics of intergrain boundaries dominate the transport processes in these polycrystalline films.