A method of alumina and silica sols precipitation in supercritical CO 2 has been developed for the first time to prepare alumina and silica doped alumina. The silicon content of samples is varied within the range from 0 to 30 mol. %. The approach suggested can be considered as a development of supercritical antisolvent (SAS) method that is widely used to prepare powders from solutions but not sols. The alumina and silicon-doped alumina obtained are shown to be mesoporous materials with narrow size distribution in the range of 3–5 nm. The high-temperature treatment of the sample containing 10 mol. % of Si results in the formation of γ-Al 2 O 3 phase followed by its transition into the δ-Al 2 O 3 phase at 1200 °C without α-Al 2 O 3 phase formation. It is demonstrated that the gradual introduction of silica into the alumina allows one to fine-tune the strength and amount of acid sites existing in oxides.