Multiphase triblock styrene-b-(ethylene-cobutylene)-b-styrene (SEBS) copolymers chemically modified with maleic anhydride (MAH) in the presence of a radical initiator by reactive extrusion were studied by solid-state H-1-NMR and C-13-NMR. In the experiments performed, the concentrations of MAH and initiator were kept constant, whereas the temperature profile in the extruder was varied. Samples with known extents of grafting and crosslinking were analyzed with NMR with techniques based on proton spin diffusion to investigate the microphase structure of the modified copolymers. The C-13-NMR results show that the size of the rigid domains was about 15 nm and was not significantly changed by the modification. Alterations in the rubbery phase were illustrated by measured changes in proton spin-spin (T-2) relaxation times. The fraction of protons having intermediate mobilities increased slightly in modified SEBS with respect to that observed in unmodified copolymers. These results were found to be independent of the extruder temperature profiles used, at least in the range studied. (c) 2007 Wiley Periodicals, Inc.