The evolution of the tantalum oxide structure during low pressure thermal oxidation of epitaxial Ta films on a sapphire substrate was investigated. Thin Ta films were deposited using magnetron sputtering on sapphire substrates at a temperature of 700 degrees C. Thermal oxidation of these films in oxygen at a pressure of 1.0 Pa at a temperature of 700 degrees C produced epitaxial Ta2O and Ta2O5 films as determined by x-ray diffraction techniques. The epitaxial Ta2O film had a cubic structure with a (101) plane oriented in the substrate plane. The epitaxial Ta2O5 films had a twinned orthorhombic structure with a (201) plane oriented in the substrate plane.