The paper presents the results obtained in the mullite synthesis using the sol –gel method, starting from Si(OC₂H₅)₄ (TEOS) and aluminum nitrate, respectively the sol-gel method combined with the combustion method. The combination of the two methods was achieved by addition urea, corresponding to the reaction with aluminum nitrate. The initiation of the combustion process was made by introducing the mixtures in the oven pre-heated at 500°C. The mullite formation requires further annealing at 1200°C, so in the case of the sol-gel method and in the combined version. The major difference between the two methods consist in avoiding the  NO_x release by using the combined version.