The reaction of oxygen with the chlorides of Si and Ge to form Si1-xGexO2is the basis for the preparation of controlled refrac tive-index glasses from which fibers for optical communications are drawn. Knowledge of the chemical species identity and concentration predicted from thermodynamics at equilibrium can provide the basis for further understanding of the kinetics of the reaction. The thermochemistry and equilibrium species concentration ale examined in this paper using the method of element potentials-a newly applied and highly effective method of treating complex equlibria. We have critically chosen and assembled pertinent thermodynamics data for the important species found in the oxidation and have examined the species concentration over the appropriate temperature and concentration range. The Gibbs free energy for SiO2-GeO2glass was calculated assuming it to be a nearly ideal solution. The element potential method combined with techniques, such as steepest descent, for finding the system free-energy minimum, enables one to deal with as many as 30 species simultaneously.