[en] ²³Na nuclear magnetic resonance (NMR) measurements on poly (propylene oxide) (PPO) and siloxane based polymer electrolytes containing various sodium salts at a single nominal concentration are reported. In addition, differential scanning calorimetry (DSC) and electrical conductivity studies were carried out on the PPO materials. The NMR-determined mobile Na⁺ concentrations and DSC results provide evidence for ionic aggregation effects which, for some samples, result in salt precipitation at elevated temperatures. ²³Na chemical shifts observed in solid state NMR due to mobile Na⁺-anion interactions influence ionic transport as well as the number of available carriers. (author). 19 refs.; 7 figs