The protectiveness of mullite layers electrophoretically deposited on C/C-Si-SiC composites, against isothermal oxidation in air in the temperature range from 1200 to 1550 °C, was investigated by means of thermogravimetry (TG). The experimental results are interpreted with the help of a phenomenological model. At lower temperatures or short oxidation times the overall oxidation kinetics is determined by transport processes in the EPD mullite layer, which leads to a linear growth law. At higher temperatures or longer times of oxidation the oxidation rate is controlled by solid-state diffusion processes in the growing silica layer, which leads to a parabolic growth law. Comparison of experimental parabolic and linear rate constants with calculated ones suggests, in the framework of the model, the conclusion that carbon monoxide (CO) diffusion in the oxide layers is the rate determining step for the overall oxidation of the C/C-Si-SiC substrates.