The optical and electrical characteristics of spray pyrolysis deposited aluminum oxide films are reported. The films were deposited from a spraying solution of aluminum acetylacetonate in N,N-dimethylformamide using an ultrasonic mist generator on (100) Si substrates. The addition of water mist during the spraying deposition process resulted in an overall improvement of the films characteristics. The substrate temperature during deposition was in the 450–650 °C range. Deposition rates up to 90 Å/s were obtained depending on the spraying solution concentration and substrate temperature with an activation energy of the order of 31 kJ/mol. The optical energy band gap for these films was 5.63 eV and the refractive index at 630 nm up to 1.66 was measured by ellipsometry. The electrical characteristics of the films were determined from the capacitance and current versus voltage measurements of metal–oxide–semiconductor (MOS) structures incorporating them. A dielectric constant of 7.9, interface states density of the order of 1011×1/eV cm2 as well as breakdown fields higher than 5 MV/cm were determined in this way. © 1998 American Institute of Physics.