Added sulfur in polysulfide aircraft sealants can become chemically bound to the polymer backbone. The chemical processes involved, and their effect on thermal performance of the resulting sealants, have been examined. Reactions of aliphatic thiols with sulfur and amine catalysts have been reinterpreted to include possible involvement of hydropolysulfide intermediates. When the procedure was applied to polysulfide liquid polymers, products with substantial levels of tri- and tetra-sulfide links were formed. C-13-NMR spectroscopy of model compounds enabled the assignment of chemical shifts associated with S-3 to S-5 links in the sulfur enriched polymers. Spectroscopic examination afforded no evidence for the presence of such species in commercial polysulfide liquid polymers. No obvious detrimental effects on elevated temperature performance of polysulfide sealants resulted from the incorporation of up to 1% sulfur into the polymer backbone. A higher degree of cure is achieved and this is associated with increased hardness and modulus, together with reduced extension as compared with untreated sealants.