The relationship between the structures and properties of solid polymer electrolytes formed by poly(tetramethylene succinate) and alkali metal salts is investigated in this article. The complexation between ions and polar groups, e.g., ester groups in the polymer chains, is determined by FT-IR and XPS techniques. The complexes are semicrystalline materials with the crystallites similar to pure polyester. Inorganic salts do not enter the crystal lattice of the polymer, whereas they do mainly dissolve in the amorphous domains of the polymer. The addition of salts depresses the melting temperature and crystallinity of the polyester. On the contrary, it increases the glass transition temperature. The conductivity of the polyester complexes varies with the concentration, species, solubility of salts in the polyester, temperature, etc. Ionic migration in the amorphous regions of the polymer contributes to the conduction of the electrolytes to a great extent. The conductive behavior could not simply be described either by the Arrhenius equation based on classical theory or by the WLF equation based on free volume theory.