Some electrochemical properties of polycrystalline tin oxide in the form of thin films have been examined. For a variety of antimony‐doped specimens the carrier concentration has been determined from the space charge capacity and compared with donor concentrations. Disparities between the two values have been found, and explanations have been sought in terms of structural defects as well as chloride impurity and nonstoichiometry. Analyses for the last two have been made. The kinetics of several reactions at highly doped specimens indicate that the space charge region is completely transparent to electrons due to tunneling. With a series of samples of diminishing carrier concentration the increasing limitation of the tunneling process is demonstrated until it becomes the principal current controlling factor at about .