The present application describes a porous polymethylsilsesquioxane (PMS) having a specific surface area between approximately 50 m2/g and 500 m2/g, a pore volume between approximately 0.1 cm3/g and 0.8 cm3/g, a monolayer volume between approximately 10 cm3/g and 60 cm3/g, a pore radius smaller than 1 nm for 90% of the pores and insolubility in water and organic solvents. The PMS according to the invention is prepared by precipitation of a polymethylsiliconate of the general formula in which n is a whole number between approximately 20 and 100 and M is an alkaline metal or alkaline-earth metal or NH4+.