The present invention is in the modification of starch by silanes and is performed by bringing the starch into intimate contact with the hydrolyzates of the silanes in the presence of alkali aluminates or alkali hydroxides. Preferably this is done in the aqueous solution of the silane hydrolyzate, in which case the additives can be used either in aqueous solution or in solid form. The aluminate or alkali hydroxide is present at least in such an amount that 0.4 mol of aluminate will be present for each mole of silane. The modification is possible not only with alkyl alkoxysilanes, but also with organofunctional alkoxysilanes. The starch modified with the silanes has better practical properties than corresponding unmodified starch and can be used, for example, for the hydrophobation and oleophobation of cellulosic material.