Abstract We have investigated the thermal and ionic conductivity properties of the elastomer poly(ethylene oxide- co-epichlorohydrin) filled with NaI and I 2. The reason for using this composition is its potential application as electrolyte in photoelectrochemical cells. This copolymer was characterized as a function of NaI concentration, temperature and relative humidity. According to the data obtained, the Na + ion interacts with the ethylene oxide repeating units by means of Lewis type acid–base interactions. The empirical Vogel–Tamman–Fulcher equation was used to model the conductivity and temperature relationships, indicating that the conduction occurs in the amorphous phase of the copolymer. The sample with 9.0% (w/w) of NaI presents a conductivity of 1.5×10 −5 S cm −1 in a dry atmosphere (30°C, [H 2O]<1 ppm) and 2.0×10 −4 S cm −1 at 86% relative humidity (22°C).