With a small amount of isocyanate-terminated polysulfide (NCO-PSF-NCO) as additive, the commercial polysulfide-based sealant had significant improvement in mechanical properties. The test results showed that 5% of NCO-PSF-NCO could help to rise 50%, in the tensile strength and 27% in the ultimate elongation of the polysulfide sealant. The adhesive strength to substrates such as steel, epoxy-coated or polyurethane-coated surface also increased significantly. Dynamic mechanical thermal analysis and differential scanning calorimetry analysis revealed that formed polysulfide polythio-urethane-urea (polysulfide-urea) contributed to a slight increase in the T, of the polysulfide sealant. It might be due to the domains of polysulfide-urea, which had effects of physical crosslinking and chain-extending on the polysulfide sealant. However, the introduction of polysulfide-urea structures had no appreciable influence on oil resistance of the polysulfide sealant. (c) 2007 Wiley Periodicals, Inc.