The chemical reduction of ultrathin oxides in aluminum-SiO2-silicon metal oxide semiconductor (MOS) structures during post-metalization anneal was Studied by means of electrical measurements. This reduction is a possible way of increasing the capacitance of the MOS device without necessarily increasing the leakage current and also without harming the electrical Si-SiO2 interface. This is accomplished by replacing part of the SiO2 with a compound of higher permittivity. The reduction was found to be insensitive to (i) the substrate doping type, (ii) the oxidation ambient, (iii) the oxidation temperature, and (iv) the initial oxide thickness. It appears to be dependent on the oxide quality. An accurate method of determining the thickness difference between two dielectrics using capacitance-voltage measurements is also presented.