FT-Raman (Fourier transform Raman) and FTIR (Fourier transform infrared) studies, along with DTA (differential thermal analysis) and XRD (X-ray diffraction) measurements, were performed on PEO-NaSCN-urea/thiourea solid polymer electrolytes (SPEs) at room temperature, and the interactions of urea/thiourea with NaSCN, PEO, and PEO-NaSCN were analyzed respectively. It is shown that the eutectic molten salt of urea-NaSCN occurs because of the interaction of sulfur in SCN- with the complex cation formed by the coordination of the N atom in urea with Na+. Urea and thiourea can form complexes with PEO by hydrogen bonds. When urea is added to PEO-NaSCN SPEs, the formation of such hydrogen bonds and the molten salt of urea-NaSCN causes crystalline PEO and the P(EO)(3)NaSCN complex to break up and results in significant changes of the ion association components. In contrast with urea-NaSCN mixtures, the formation of molten salt in thiourea-NaSCN mixtures was not observed and the interactions of thiourea with NaSCN were found to be so weak that the crystal P(EO)3NaSCN complex could not be broken up in the PEO-NaSCN-thiourea systems. However, the formation of hydrogen bonds and low-melting complexes in the thiourea-PEO systems improves their compatibility and makes the transformation of PEO from the crystalline to the amorphous form possible.