The ionic conductivity of highly deproteinized liquid natural rubber having epoxy group (LEDPNR) mixed with alkali metal salts was investigated through impedance analysis to clarify the effect of proteins present in the rubber. The LEDPNR, thus used, was prepared from depolymerization of epoxidized natural rubber (ENR) latex, which was deproteinized by incubation of the latex with proteolytic enzyme and surfactant. The ionic conductivity of the resulting LEDPNR was dependent upon the alkali metal salts, where the ionic conductivity of LEDPNR/bis(trifluoromethane sulfonyl)imide (LiTFSI) was higher than that of LEDPNR/lithium perchlorate (LiClO4). The difference in the ionic conductivity was attributed to the solubility of the salts through both high-resolution solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy and measurements of spin-lattice relaxation time. The ionic conductivity of LEDPNR/LiTFSI was also dependent upon concentration of LiTFSI and the conductivity reached the highest value at 20 wt.%, which was different from the monotonic increase in the ionic conductivity of liquid ENR prepared from untreated natural rubber latex. (C) 2004 Elsevier B.V. All rights reserved.