The poly(ethylene oxide)-based solid electrolytes M[N(CF3SO2)(2)](2)PEOn with M = Mg, Ca, Sr, and Ba and n = 6-24 have been examined with proton NMR spectroscopy, DSC, and X-ray diffraction to explore some of their dynamic and structural properties. Changes in salt concentration appear to be more important than changes in ionic radius for properties such as activation energies and correlation times for conformational transformations. Above the glass transition temperature, the fully amorphous samples with M = Mg and n less than or equal to 12 produce proton NMR spectra that can be decomposed into two components of different width. This is a consequence of the different dynamic properties of cation-coordinated and noncoordinated polymer chain segments. The relationship between the areas of these components results in a maximum effective coordination number for the Mg2+ ion between 6 and 7, depending on salt concentration. Some of the initially amorphous samples undergo slow crystallization, producing aged samples with crystalline PEO-salt complexes.