Fabrication of mullite (3Al(2)O(3)(.)2SiO(2)) coatings by chemical vapor deposition (CVD) using AlCl3-SiCl4-H-2-CO2 gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl3-SiCl4-H-2-CO2 system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings similar to2 mum thick were successfully grown on Si3N4 substrates at 1000degreesC and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains similar to0.3-0.5 mum in size.