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The electron trapping in MOS capacitors with amorphous Al2O3 as an insulating layer was studied through pulsed capacitance-voltage technique. A positive shift of the voltage value corresponding to a constant capacitance (VC) was observed. The dependences of the voltage instability with the applied bias and the charging time were investigated. Two different contributions could be distinguished: a hysteresis phenomenon observed on each measurement cycle, and a permanent accumulated VC-shift to which each measurement cycle contributes. A physical model based on tunneling transitions between the substrate and defects within the oxide was implemented. From the fitting procedure within the energy range covered in our measurements (1.7?2.7 eV below the conduction band edge), the trap density was found to decrease exponentially with trap energy depth from 3.0 × 10²⁰ cm−3 eV⁻¹ to 9.6 × 10¹⁸ cm−3 eV⁻¹, with a uniform spatial distribution within the first 2 nm from the semiconductor interface for the hysteresis traps.