A study of charge injection in metal-Al2O3-Si02-silicon (MAOS) structures has been made using the capacitance-voltage technique. Thin SiO2 films were grown thermally on the Si substrates, whereas Al2O3 layers were obtained by pyrolytic deposition from AlBr3 in a NO-forming gas mixture. Independent of the electrode material, negative charge is introduced into the oxide system when a positive voltage is applied to the metal. With Al electrodes, the oxide also becomes negatively charged under negative bias. Only for large negative voltages will a gradual loss of negative charge occur, leading to a net positive oxide charge. Charge injection under negative bias is not observed on O2 annealed Al2O3 layers, or on samples with Au electrodes. The results are discussed in terms of a qualitative model. © 1971, The Electrochemical Society, Inc. All rights reserved.