A method of preparing a hydrophobic mineral and/or filler material, for example a gypsum material, comprises (i) preparing a hydrolyzate containing polysiloxanes by hydrolyzing a hydrolysable silane or hydrolysable silane mixture in the presence of an acid hydrolysis catalyst; and (ii) combining the polysiloxane hydrolyzate with the mineral and/or filler, and optionally water and/or a catalyst for condensation of the hydrolysate.