In poly(ethylene oxide) (PEO) based solid polymer electrolytes, the interaction between cations and the ether oxygen plays a major role in ion conductivity. Measurements with differential scanning calorimetry (DSC) illustrated clearly the modification of the PEO crystalline structure with increasing content of magnesium salt. FTIR spectral studies suggest interaction of Mg2+ cations with the ether oxygen of PEO, where a 1100 cm(-1) broad band corresponds to COC stretching and severe deformation occurs. A spectral band at similar to623 cm(-1) corresponds to the ClO4- anion and shows the growth of a shoulder at a higher wave number with increasing salt content. The apparent new envelope at similar to634.5 cm(-1) clearly indicates ClO4--Mg2+ ion pairing. Ionic conductivity increases with salt content, and is optimized at 15 wt.% Mg salt (O:Mg ratio 28: 1). The decrease in ion conductivity at higher salt contents is due to ion-ion association, which leads to ion pair formation (i.e. aggregation of ionic salt) and retards the motion of ions.