A hydrothermally stable, microporous organic-inorganic hybrid membrane based on silica, having an mean pore diameter of between 0.2 and 1.5 nm, is characterised in that between 5 and 40 mole % of the Si—O—Si bonds have been replaced by moieties having the one of the formulas: Si—{[CmH(n-1)X]—Si—}q, Si—[CmH(n-2)X2]—Si or Si—CmHn—Si{(CmHn)—Si—}y in which m=1-8, n=2m, 2m−2, 2m−4, 2m−6 or 2m−8; provided that n≧2, X=H or (CH2)pSi, p=0 or 1, and q=1, 2, 3 or 4. The membrane can be produced by acid-catalysed hydrolysis of suitable bis-silane precursors such as bis(trialkoxysily)alkanes, preferably in the presence of monoorganyl-silane precursors such as trialkoxy-alkylsilanes.