The referent film of poly(ethylene oxide)/sodium alginate blend with 80 wt% of poly(ethylene oxide) and 20 wt% of sodium alginate (80%PEO/20%NaAlg) was prepared by solution casting method. The cross-linkage of sodium alginate in the blend film with Ca 2+ ions was conducted with 1, 2.5, 5, 7.5 and 10% w/v CaCl 2 methanol solution during 10, 30, 60 and 180 minutes exposure time after which the films were rinse twice. The structural change of NaAlg was investigated by Fourier transform infrared spectroscopy (FT-IR) according to which the cross-linkage with Ca 2+ ions was successful. Differential scanning calorimetry (DSC) has been applied to investigate the influence of cross-linkage treatment to the thermal properties and crystallinity of PEO in the films. The cross-linkage with Ca 2+ ions has significant effect on the crystallization process of PEO and under the specific conditions it lowers the amount of crystalline phase of PEO in the treated films. Thermal stability of the treated films was investigated using non-isothermal thermogravimetry (TG). The specific conditions make the degradation of alginate in the treated films more stable while at the same time they decrease the rate of the degradation of PEO. All analysis confirmed the successful cross-linkage with Ca 2+ ions. In the future investigations cross-linkage of NaAlg with Ca 2+ ions in 80%PEO/20%NaAlg film will be carried out only at the highest concentration of CaCl 2 methanol solution (10% w/v) and the highest exposure time to the solution (180 min). These conditions showed the best results and obtained film under these conditions has the greatest potential to be a solid polymer electrolyte.