The treatment of bovine leathers with wetting and lyotropic agents followed by heating produced a strengthening of the leathers which increased their joint strength properties to polyvinyl chloride (PVC). A cohesive failure of leather was always obtained. The highest cohesive strength (or point peel strength) was obtained when the treatment was carried out at 140-degrees-C with the surfactant NFOE (8.5) (nonylphenol polyoxyethylene with 8.5 mol of oxyethylene). The lyotropic agents (CaCl2, urea) gave very high values (a five-fold increase), whereas the water-dimethyl ketone blends and pure water resulted in a smaller improvement in cohesive strength (a three-fold increase). The improved cohesive strength of leather was mainly due to the destruction of the ordered structure of collagen fibres and to the creation of a complex entanglement network among the collagen fibres. The treatments applied to a bovine leather produced a shrinkage of 65%; the degree of shrinkage was not a function of the kind of treatment, but of the structure of the leather. The application of surface treatments to leather prior to its bonding to other substrates may mean that the roughening process of the leather, a tedious and difficult operation which is necessary in order to obtain adequate adhesive joints, can be avoided.