The ceramic coatings are excellent candidates to protect metallic structures that work at high temperature. Inside these ceramic coatings, the mullite is a good option, since it presents very good mechanical properties, great corrosion resistance, high thermal resistance and high durability. The CVD-FBR (Chemical Vapor Deposition by Fluidized Bed Reactor) is an interesting technique to create adherent films on metallic surfaces to protect them. Furthermore, this method is cheap and easy to apply, and the most important characteristic is that could be applied at much lower temperatures comparing for example to pack cementation. The first step to obtain mullite coatings would be the co-deposition of aluminium and silicon coatings by CVD-FBR to decrease the thermal mismatch between substrate and coating. Thermodynamic calculations were made before experiments to study the oxidation system and optimize the working conditions using Thermo-Calc code. These depositions take place in a fluidized bed reactor. The base material used was an AISI 304 stainless steel. This technique is based upon the reaction among aluminium chloride (AlCl3(g)) and silicon chloride (SiCl4(g)), among other precursor species. The optimization of the conditions (deposition temperature, time, fluxes, etc) is discussed in the present work. The coatings design is completed with diffusion simulations during the CVD process and subsequent heat treatment. The analysis of results is carried out by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) methods. In addition further oxidation of these precursor coatings is made in order to obtain the definitive system of protective ceramic layer. The oxidation of coating samples is made at different temperatures and time conditions to obtain the best mullite structure, based on predicted thermodynamic calculations by Thermo-Calc.