Polypentenamer has been sulfonated using a 1:1 complex of sulfur trioxide and triethyl phosphate in chloroform. A range of polymers containing from 1.9 to 19. l mo! % sulfonate groups was prepared by this reagent and isolated in the form of sodium salts. These materials are initially free from covalent cross-links but cross-linking occurs with aging unless the polymers are stabilized with a free-radical scavenger such as hydroquinone. The unsaturated bonds in these materials can be removed by hydrogenation with p­toluenesulfonyl hydrazide. The hydrogenated derivatives are crystalline and no longer require a stabilizer to inhibit cross-linking. Both the unsaturated and hydrogenated polymers are hydrophilic.