Abstract: A solid material is treated, the chemical structure of which defines reactive protogenic hydrophillic functions accessible to gases, by applying at least one gas stream (3) onto at least one microdispersion (5) of at least one grafting reagent RX produced on the solid material, R being a hydrophobic group, X being chosen so that HX is volatile under normal conditions, R and X being chosen so that the reaction of RX on the hydrophillic functions produces covalent grafting of the hydrophobic group R with formation of the compound HX, it being possible for the reaction to be carried out in a solid/gas heterogeneous medium on all the reactive hydrophillic functions accessible to gases and only on these. The invention extends to the hydrophobic solid material obtained, and is applicable to the obtaining of natural or artificial fibrous or inorganic structures impermeable to water and to aqueous solutions and/or absorbing fats.