Abstract: Leaks in a vessel or pipeline or the like, adapted for containing a fluid, are sealed by means of a volatilized polymerizable organosilane comprising a mercaptosilane, an acyloxysilane, or an alkoxysilane taken on conjunction either with acetic acid anhydride or an amine, or mixtures thereof, the organosilane being supplied to the interior of the vessel or pipeline under sufficient pressure and for a sufficient period of time to permit contact and chemical reaction thereof in the presence of moisture or sand at the situs of the leak to provide a solid polysiloxane seal.