Formation of mullite on ceramic substrates via chemical vapor deposition was investigated. Mullite is a solid solution of Al2O3 and SiO2 with a composition of 3Al2O3 · 2SiO2. Thermodynamic calculations performed on the AlCl3–SiCl4–CO2–H2 system were used to construct equilibrium chemical vapor deposition (CVD) phase diagrams. With the aid of these diagrams and consideration of kinetic rate limiting factors, initial process parameters were determined. Through process optimization, crystalline CVD mullite coatings have been successfully grown on SiC and Si3N4 substrates. Results from the thermodynamic analysis, process optimization, and effect of various process parameters on deposition rate and coating morphology are discussed.