Thin-oxide MOS capacitors are subjected to unipolar and bipolar dynamic voltage stresses. The characteristic times of the measured current transients indicate that they are due to changes in the occupancy of interface states. A reasonable picture is presented which invokes the restoration of equilibrium at the oxide/semiconductor interfaces, and the stripping of electrons by Fowler-Nordheim tunnelling when the applied field is very high. On the other hand, time-to-breakdown data show that, in agreement with the results of other authors, oxide reliability is considerably higher under high frequency bipolar stresses. A smaller dependence of time-to-breakdown on the frequency of unipolar stresses is also found, and is qualitatively interpreted in terms of the average energy required to breakdown. © 1995.