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It is shown that changes in the capacitance-voltage characteristic of a metal-insulator-semiconductor structure provide a powerful tool for the observation of ion motion in thin insulating films. Using this method, a detailed study of the kinetics of alkali ion migration in thermally grown silicon dioxide films has been made. Alkali ions were initially deposited at the metal-oxide interface and their transport through the oxide was studied as a function of time, temperature and applied voltage. When the metal is biased positively the number of ions accumulated at the oxide-silicon interface is initially proportional to the square root of time and then approaches a saturation value. The temperature dependence is exponential and leads to an activation energy for the diffusion coefficient of 32 kcal/mole for Na and 22 kcal/mole for Li. A simple model is developed which is based on the division of the insulator into two regions: a thin boundary layer near the metal-insulator interface in which ion transport is by diffusion, and the remainder of the insulator where field transport dominates. It is shown that most features of this model are in excellent agreement with the experimental results.