Well-characterized linear ethylene-butene-1 copolymers with polydispersities lower than 1.1 were modified using an organic peroxide with the purpose of assessing the relative importance of chain-linking processes. The copolymers used in this work were obtained by hydrogenation of polybutadienes, which were synthesized by anionic polymerization of butadiene. All the copolymers were modified in the molten state using 2,5-dimethyl-2,5 di(terbutyl peroxy)-hexane with concentrations varying on a ample range. The changes in the molecular structure of the polymers due to the chemical attack of the peroxide were evaluated combining selective solvent extraction and GPC-MALLS techniques. As expected, the critical concentration required for the onset of gelation, C-c, decreases with average molecular mass of the original copolymer. When concentrations of peroxide below C-c are employed, the weight-average molecular mass of the modified copolymers augments and the molecular mass distribution gets wider as the concentration of peroxide added to the copolymer increases. Although, the chain-linking reactions govern the modification process, there is a fraction of molecular chains that suffers scission. The measurable fraction of molecules having molecular mass lower than the original quasi-monodisperse copolymers ranges between 15% and 24% of the total mass when the copolymers were modified with concentrations of peroxide below C-c. Beyond the onset of gelation, the gel amount increases continuously with the peroxide concentration added to the copolymers. At a given peroxide concentration, the amount of gel produced for a given polymer is higher the greater is the initial molecular mass of the modified polymer. The limiting amount of gel was between 90% and 98% of the total mass, which was reached using concentrations 20 times larger than the critical one. The efficiency of the peroxide as cross-linking agent seems to be dependent on the concentration of peroxide. (C) 2005 Elsevier Ltd. All rights reserved.