A method of alumina and silica sols precipitation in supercritical CO2 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 gamma-Al2O3 phase followed by its transition into the alpha-Al2O3 phase at 1200 degrees C without delta-Al2O3 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. (C) 2019 Elsevier B.V. All rights reserved.