lntercalation of poly(ethylene oxide) (PEG) into montmorillonite results in a nanocomposite polymer electrolyte potentially useful for solid state secondary batteries. We have used solid state NMR to probe cation dynamics on the timescales relevant to spatial diffusional motion of Li+, Na+, Rb+ and Cs+ in these nanocomposites. Cation dynamics is correlated to the charge-carrying capacity of the material as an electrolyte. Unlike linewidth-narrowing studies on more traditional solid polymer electrolytes, in our systems the largest source of dipolar fields arise primarily from paramagnetic impurities fixed in the silicate lattice.