Surface of silicon oxide (white black BS-120) was hydrophobized with a polymethylhydrosiloxane solution. IR spectroscopy demonstrated that the hydrophobization process involves free silanol groups of the silicon oxide surface. It was found that a stable superhydrophobic state of the surface is observed upon hydrophobization of silicon oxide, with the wetting angle exceeding 150°. Introduction of hydrophobized silicon oxide into a fire-extinguishing powdered formulation based on ammonium phosphates can improve its rheological characteristics. The influence exerted by the fraction composition of dispersed systems based on phosphorus-ammonium salts on their rheological properties was examined.