Annealed Cu-Al-O films showed marked structural changes and differing optoelectronic properties with varying annealing temperature. Results of X-ray diffraction (XRD) demonstrated that CuO and CuAl2O4 were the intermediate reaction phases. XRD also showed that the phase grown above 800 degrees C annealing temperature was pure CuAlO2 phase. Cross-sectional high-resolution transmission electron microscopy revealed that the crystallization behavior of the Cu-Al-O films belonged to an outward model. The optimum properties of delafossite structure CuAlO2 film was attained after annealing at 800 degrees C. The surface morphology of CuAlO2 had a cell-like surface appearance and the grain sizes were approximately 20-100 nm. The optical direct bandgap of the CuAlO2 film was estimated to be 3.11 eV. Hall effect measurements revealed that the CuAlO2 film belonged to the p-type conduction category, with a carrier concentration of 4.81 x 1016 cm(-3) and the conductivity of 3.8 x 10(-2) ( Omega cm )(-1). The optoelectronic properties of the Cu-Al-O system are dominated by the delafossite CuAlO2. (c) 2007 The Electrochemical Society.