The present paper reports investigations on the application of surfactants in the stone conservation. It is found that alkyl ammonium chlorides reduce the hygric dilatation of clay-rich sandstones to half of the original value. Polysiloxane films can be tightly bound to clay mineral basal planes using aminoalkyl silane as primer. The water uptake as well as the hygric swelling are markedly reduced. Well defined mixtures of ethanolic alkyl silane and aqueous aminoalkyl silane solutions form stable microemulsions. They can be used as commercial conservative products which are compatable with the environment.