We describe the influence on the conductivity of the mixed-valence tetrathiafulvalene (TTF)/poly (methyl methacrylate) (PMMA) composites by incorporating heterogeneous TTF derivatives. We prepared the uniformly dispersed PMMA composites from the mixtures containing TTF cation radical and the series of neutral heterogeneous TTF derivatives. From the observation with a scanning electron microscope, it was found that the wire structures were formed in the PMMA films in the presence of heterogeneous TTF derivatives. In addition, the mixed-valence states were observed from the absorption measurements. The significant relationship between the conductivities of the composite films and the differences in oxidation potentials between heterogeneous TTF pairs was observed. The conductivities were observed at 10(-2) S cm(-1) order from the PMMA composites containing TTF cation radical and heterogeneous TTF derivatives which have similar oxidation potentials. In contrast, the conductivities of the PMMA composites were found at 10(-4) S cm(-1) order with the gaps of the oxidation potentials between heterogeneous TTF pairs. These results suggest that the TTF derivatives which possess larger oxidation potentials than those of the other coexisting TTF derivatives could work as an energy barrier.