Nanocomposites based on poly(styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS) and multiwalled carbon nanotubes (MWNTs) were prepared. In order to facilitate the dispersion of MWNTs in the nanocomposites, the pristine MWNTs were reduced by Li to anions and SEBS was sulfonated. Poly(4-styrenesulfonic acid) (PSSA) was also added to either SEBS or S-SEBS and the morphology and properties of the nanocomposites were examined. Nanocomposites prepared from S-SEBS, PSSA, and the modified MWNTs showed a threshold decrease in electrical resistivity due to the percolation of MWNTs at a critical MWNT content (P(c)) lower than those for the nanocomposites based on SEBS, PSSA, and the MWNTs. It was found that the P(c) decreased with increasing the concentration of PSSA in the nanocomposites containing S-SEBS. The efficient percolation of MWNTs in the S-SEBS/PSSA nanocomposites was attributed to interactions between the ionic moieties of MWNTs and S-SEBS and the fine dispersion of the MWNTs. Nanocomposites based on S-SEBS showed large increase of rubbery plateau modulus compared with those based on SEBS. PSSA played the role of plasticizer of sulfonated PS blocks above T(g) of PS blocks in S-SEBS.