A systematic study of flatband voltage transients occurring in high-k dielectric-based metal-insulator-semiconductor structures has been carried out. Materials under study have been HfO2, HfSiOx, Gd2O3, Al2O3, and TiO2 deposited on n-type silicon. At the flatband condition, in samples measured in darkness, under no electric fields and no field-related charge injection conditions, these transients are due to localized states in the insulator trapping electrons injected from the semiconductor by tunneling. The process is assisted by phonons and it is therefore thermally activated. Dependencies of the flatband voltage transients on the dielectric material, the bias history, and the hysteresis sign of the capacitance-voltage curves are demonstrated. The magnitudes of the flatband voltage transients clearly depend on the layer crystallinity; amorphous materials exhibit much lower transient amplitudes than crystalline or polycrystalline dielectrics. (C) 2008 The Electrochemical Society.