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The nanocomposites based on poly (styrene-b-ethylene butylene-b-styrene) (SEBS) and multiwalled carbon nanotubes (MWNTs) were prepared and electrical, thermal, and mechanical properties of the nanocomposites were investigated. SEBS is a block copolymer showing microphase separated morphologies and can be a matrix polymer for double percolation of MWNTs in nanocomposites. In order to facilitate the dispersion of MWNTs, pristine MWNTs were reduced with Li into anions and sulfonation of SEBS to prepare sulfonated SEBS (S-SEBS) was carried out. Furthermore, we have added poly (4-styrenesulfonic acid) (PSSA) to SEBS or S-SEBS and investigated properties of the nanocomposites based on the MWNTs, SEBS, and PSSA. Nanocomposites based on S-SEBS, PSSA, and the modified MWNTs showed threshold decrease of electrical resistivity due to the percolation of MWNTs at a lower content of the MWNTs compared with those based on SEBS, PSSA, and the MWNTs. The efficient percolation of the MWNTs in the nanocomposites of S-SEBS and PSSA is attributed to interactions between ionic moieties of MWNTs and S-SEBS and fine dispersion of the MWNTs. It was worthwhile to notice that the addition of PSSA into S-SEBS resulted in the decrease of the critical concentration of MWNTs (Pc) for the percolation in the nanocomposites. It is postulated that the decrease of Pc resulted from a double percolation of MWNTs by preferred location in sulfonated polystyrene phases of microphase separated S-SEBS in the nanocomposites. The nanocomposites based on S-SEBS showed large improvement of Young's modulus compared with the nanocomposites based on SEBS.