In the widely used modified chemical vapor deposition (MCVD) process for making optical fibers, germanium is the primary dopant for precisely increasing the refractive index of silica to form a guiding structure. The reactants in the process are usually GeCl4, SiCl4, and O2, often with POCl3, flowing in a silica support tube heated from the outside with an oxy-hydrogen flame. The oxide reaction products which are formed in the hot zone deposit as small particles downstream on the walls of the tube, and are consolidated into clear glass layers as the hot zone is traversed in the direction of flow. We have investigated the chemistry of germanium incorporation in the glass by infrared spectrophotometric analysis of the effluent gases, and by measurement of the properties of preforms and fibers produced by the process. It was found that SiCl4oxidizes completely to form the base glass, but GeCl4is incompletely reacted, with as much as 70 percent of the initial germanium present in the effluent as GeCl4. This is due to an unfavorable equilibrium in the reaction of GeCl4with O2to form GeO2, and we have developed a simple model for the process based on this equilibrium. At low temperatures the reactions in the hot zone are controlled by kinetics, but at the higher temperatures encountered in practice the gas concentrations and glass composition are controlled by thermodynamic equilibria. The composition of the glass produced can be quantitatively predicted by the model from such process parameters as the starting reactant concentrations, and the gas flow rates. It is necessary to include the dynamics of particle formation in the model to understand the inhomogeneity of multilayered deposits and the dependence of the final composition on the reaction zone maximum temperature. This results from the fact that the particles may grow large enough, especially with phosphorus present, to prevent diffusion of Ge from the particle interior to the equilibrating ambient at the surface. The model has been useful for predicting process parameters for the production of fibers with tightly controlled composition profiles.