The necessity for good adhesion in sealants and the mechanism of adhesion development in polysulphide sealants are discussed. The studies reported give the effect of co-reacting polysulphide resin with epoxy resin on the adhesion of a polysulphide sealant composition to an aluminium substrate and also on selected mechanical properties. The sealant was formulated from a mercaptan-terminated liquid polysulphide polymer. The liquid polymer was co-reacted with epoxy resin before formulating the sealant. Modification with epoxy resin improved the adhesion of the sealant. Adhesion is improved because of the presence of hydroxyl groups in the chain after co-reaction. The hydroxyl groups increase the ability of the polymer to remove the loosely adsorbed water layer present on the surface and establish strong interfacial forces with the surface.