Polar polymers containing alkali metal salts, function as solid electrolytes. The conductivity of these materials is greatest when the polymer has a high degree of segmental motion, as indicated by a low value of T//g. Comblike polymers with short-chain polyether side chains on a polyphosphazene backbone have low values of T//g and these form polymer salt complexes with good conductivity. The conductivity is however sensitive to composition of these polymer salt complexes. For example, the conductivity goes through a maximum as a function of increasing salt concentration and the conductivity is strongly influence by the structure of the side chains. Recent **1**3C NMR spectroscopic give some insight into the mode of coordination of the alkali metal to the side chains and the role of side chain structure in determining conductivity.