The efficiency of a perfluoropolyether as water repellent has been experimented on samples of Carrara Marble and Pietra Serena (a sandstone). Such a treatment reduces almost completely the penetration of water in marble. The same treatment reduces drastically also the penetration of water in Pietra Serena. Also the rate of diffusion of the humidity inside the sample is considerably reduced. The loss of humidity from the samples by evaporation is not affected by the treatment with this water repellent. The smoothness of the surface appears to affect considerably the efficiency of the treatment.