Mobile ions in SiO2layers of MOS structures have been investigated with the thermally stimulated ionic current (TSIC) method. Two distinct peaks were observed in the temperature range 30–350 °C, the first at about 150 °C and the second at about 300 °C. Measurements on samples intentionally contaminated with either sodium or potassium show that the first peak is due to the motion of sodium ions, whereas the second peak results from the motion of potassium ions. The surface‐trapping model describes the curves well when a Gaussian spread in activation energy is assumed. When a physically plausible value for the emission time constant is assumed (10−12s), an activation energy is found that is consistent with an emission‐limited process.