The modification of poly(ethylene oxide) (PEO) of different molecular weight was carried out with addition of sodium alginate (NaAlg) by the extruder and the press, to obtain better thermal characteristics and crystallinity of PEO. Only some PEO/NaAlg composites with different molecular weight of PEO and with extrusion and compression have been successfully prepared. Fourier transform infrared spectroscopy (FT-IR) did not confirm the interaction between PEO and NaAlg in the form of hydrogen bonds. Using differential scanning calorimetry (DSC), the effect of different proportions of biodegradable polymer from renewable sources (NaAlg) on the thermal characteristics and crystallinity of water-soluble PEO with different molecular weights was investigated, as well as the dependence of these characteristics on the preparation process. The results indicate that NaAlg does not change the thermal characteristics and crystallinity of PEO. The change in molecular weight of PEO has no significant effect too, but the influence of different preparation processes cannot be neglected. The thermal stability of the composite was investigated using nonisothermal thermogravimetry (TG). PEO/NaAlg composites decompose in three degradation stages. NaAlg impairs the thermal stability of all composites and no difference was found between the influence of different molecular weights and the preparation processes.