The oxide composition and the electronic behavior of anodically and thermally oxidized SiGe single crystals and epitaxial layers have been investigated. X-ray photoelectron spectroscopies show the existence of Si4+ and Ge4+ only in the anodic oxides, whereas the thermally grown oxide layers on SiGe consist of pure SiO2. Infrared spectroscopy reveals the existence of Ge as part of mixed oxide compounds like Si-O-Ge and not as GeO2 up to a Ge content of 26 at.% in the SiGe lattice. Ge segregation at the oxide/SiGe interface and relaxation phenomena of the strained SiGe lattice of epitaxial layers, which occur during thermal treatments, are completely suppressed using the anodic treatment. In addition, the defect concentration at the interfaces is reduced as obtained from photoluminescence and capacitance-voltage measurements as compared to thermally grown oxides. The amount of Ge in the lattice has an influence on the defect concentration.