Results of contamination experiments on MOS structures with aqueous solutions of NaCl, KCl, acetic acid, and purified water are presented. A special grid-shaped electrode (Al or polycrystalline Si) was developed for these experiments. Various after-treatments were applied. At temperatures of 250 and 350 °C, the I-V curve shows two distinct peaks caused by ion transport in the oxide. Contamination with H+ and Na+ ions had a pronounced influence on the area of both peaks. The results showed that it is impossible to attribute the first peak to transport of Na+ ions only and the second to that of K+ ions. It is concluded that the first peak results from ion transport under conditions of thermodynamic equilibrium and that the second peak is controlled by the rate with which ions are released from traps at the interfaces. Models for both transport mechanisms are presented.