In this paper a study of the bulk functionalization of styrene-b-(ethylene-co-1-butene)-b-styrene triblock copolymer (SEBS) with diethyl maleate (DEM) or maleic anhydride (MAH) and dicumyl peroxide (DCP) as initiator in a Brabender mixer is described. The determination of the functionalization degree (ED) by NMR analysis indicates that the FD values depend on the feed composition and in particular on the DEM/DCP ratio. All obtained products were fractionated by solvent extraction and characterized by IR, NMR and GPC. The results obtained show that the functionalization takes place with a very large preference at the aliphatic carbons of the polyolefin block. Moreover occurrence of degradation and chain extension reactions gives st functionalized product with a MWD larger than 1. Acetone extraction allows the isolation of styrene rich oligomers, thus the high MW functionalized polymer has a lower content of aromatic units than original SEES. The results are described with reference to reaction mechanism previously reported in case of the functionalization under similar conditions of polymonoalkenes. (C) 2000 EIsevier Science Ltd. All rights reserved.