The invention relates to a process for preparing organosilanes of the general formula I ##STR00001## by reacting (haloorganyl)alkoxysilane of the formula II ##STR00002## with hydrous alkali metal hydrogensulphide, sulphur and alkali metal carbonate in alcohol, wherein the molar (haloorganyl)alkoxysilane of the formula II to alkali metal hydrogensulphide ratio is between 1:0.4 and 1:0.75, and the molar alkali metal hydrogensulphide to alkali metal carbonate ratio is between 1:0.5 and 1:1.5.