The invention concerns a method for preparing organic-inorganic hybrid materials with controlled porosity and functionality comprising hydrolysis and condensation polymerization of a compound of formula (I): Z-R.sub.1--Si(OR)(OR')(OR''), with an alkaline or alkaline-earth silicate in a mol ratio of (I)/(silicate)=1:9, in the presence of a non-ionic surfactant, to form a mesoporous silica represented by Z-R.sub.1--SiO.sub.1.5 9 SiO.sub.2, whereon is then anchored an organic compound, represented by (.DELTA.)N--H, comprising a --NH-- group capable of reacting with Z, to form a functionalized mesoporous silica represented by (.DELTA.)N--R--SiO.sub.1.5 9 SiO.sub.2, which is optionally complexed with a metallic cation to form an organometallic complex. The invention also concerns the use of these materials to separate gases.