Abstract: Thin SiO.sub.2 films can be produced by hydrolysis and condensation ofa) 40 to 100% by weight of one or more silanes of general formula (I)R.sub.x --Si--A.sub.4-x (I)in which the groups A are identical or different and stand for hydroxyl groups or hydrolytically separable groups, the groups R are identical or different and stand for hydrolytically non-separable groups, x has the value 0, 1, 2 or 3, x being not less than 1 for 70% by moles of said silanes;b) optionally in the presence of 0 to 50% by weight of colloidal SiO.sub.2 and/orc) 0 to 10% by weight of organic binder.The viscous sol thus obtained is worked into a gel film which is heat-treated.