The crystallinity of polyelectrolytes has long been known to affect their ionic conductivity, but the effects of water of hydration on polyelectrolyte structure are not commonly studied. Here, polymer complexes consisting of poly(ethylene oxide) (PEO) with magnesium chloride (anhydrous MgCl2, MgCl24H2O, and MgCl26H2O, respectively) have been prepared by a mixed-solvent method. Fourier transform-infrared measurements indicate each magnesium chloride salt can coordinate with PEO to form a complex. The structures of (PEO)xMgCl24H2O and (PEO)xMgCl26H2O complexes are similar, whilst the structure of (PEO)xMgCl2 complex is different to both. Wide angle X-ray diffraction studies indicate in each polymer complex system the crystallization of PEO is depressed by the interaction of magnesium cation with the ether oxygen of PEO. PEO in (PEO)xMgCl2 and (PEO)xMgCl24H2O are shown to be amorphous, but in (PEO)xMgCl26H2O it is crystalline. Polar optical microscopy images indicate in each PEO/magnesium chloride system the crystalline morphology clearly changes with the increase of magnesium salt content. The reason for the formation of the spherulites with special morphology are the strong interaction between magnesium cation and ether oxygen of PEO, and the different evaporation rates of ethanol and chloroform in mixed solvent. A better understanding of the effects of hydration on polyelectrolyte crystallinity can help in improving their use in a variety of applications. (c) 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym. Phys. 2013, 51, 1162-1174