Electron spin resonance investigations on styrene-polyisoprene-polystyrene block copolymer loaded with various amounts of multimalled carbon nanotubes are reported. The temperature dependence of resonance line parameters in the range 290-425 K was analyzed. It was proved that the main resonance line represents a bottleneck of localized and delocalized. electrons residing on carbon nanotubes. The temperature dependence of the g-factor and double integral of resonance spectra confirmed this interpretation. The temperature dependence of the resonance linewidth of composites containing various concentrations of nanotubes was explained by the thermally activated narrowing of the resonance spectra. (c) 2005 Wiley Periodicals, Inc.