The electron cyclotron resonance (ECR) plasma oxidation and thermal oxidation mechanisms for germanium were investigated using in situ spectroscopic and single wavelength ellipsometry. ECR plasma oxidation was performed at substrate temperatures of 80-400-degrees-C while thermal oxidations were performed between 400 and 600-degrees-C. Optical modeling shows that there is an interface between the oxide film and the substrate that is composed of germanium oxide and amorphous germanium. Fourier transform infrared and x-ray photoelectron spectroscopy data confirm that the oxides resulting from thermal and ECR plasma oxidation are GeO2. The real time oxidation analysis shows that both kinds of oxidation of Ge follow linear-parabolic kinetics, and yield temperature activated transport. The activation energy for ECR plasma oxidation is about one order smaller than that of thermal oxidation. This indicates different oxidation species. Less charge was formed at the interface for ECR plasma oxides.