Measurements of 1H and 7Li nuclear magnetic resonance (NMR) and ionic conductivity are reported for the polymer electrolytes formed by hydroxyethylcelluloses (HEC), poly(ethylene oxide) diisocyanate (DPEO) and LiClO4. The NMR experiments exhibit the qualitative features associated with Li+ and polymer chain mobilities in polymer electrolytes, namely the presence of well-defined 7Li and 1H spin–lattice relaxation rate maxima (T 1 −1). The single absorption band in the proton spectra and the observed 1H and 7Li exponential relaxation indicate that the HEC/DPEO/LiClO4 polymer electrolytes are predominantly amorphous. The conductivity values obtained by a.c. measurements rise above 10−4 S/cm at 370 K and are slightly higher for the samples with a greater oxygen content in the HEC molecule. The parameters estimated from the data indicate that the conductivity and the mobilities of Li+ and polymer chains are comparable to those found in the PEO8LiClO4 polymer electrolyte.