Modified natural rubber polymer hosts having low transition glass temperatures have been investigated for use in polymer electrolytes, Two types of modified natural rubber, namely 25% epoxidised natural rubber (ENR-25) and 50% epoxidised natural rubber (ENR-50) were employed in conjunction with poly(ethylene oxide), PEO. Results are reported for ionic conductivity and thermal properties for both unplasticized and plasticized polymer electrolyte systems with lithium triflate. The samples were in the form of free standing films with the thickness 0.2-0.5 mm and mixtures of ethylene carbonate (EC) and propylene carbonate (PC) were used as plasticizers. Unplasticized modified natural rubber-based systems exhibit ionic conductivities in the range 10(-6) to 10(-5) S cm(-1) at ambient temperatures. Incorporating 100% of EC/PC by weight fraction of polymer (ENR/PEO) to the systems yielded mechanically stable films and ionic conductivities in the range of 10(-4) S cm(-1) at ambient temperature.