A new class of alkaline solid polymer electrolytes (SPEs) based on poly( ethylene oxide) (PEO), potassium hydroxide (KOH), and water was investigated. The structure of the SPEs was studied by differential scanning calorimetry, thermogravimetric analysis (TGA), X-ray diffraction, and optical microscopy techniques. The existence of a crystalline complex between PEO, KOH, and H2O was evidenced for some compositions, depending on the O/K ratio. A possible structure was proposed, and a schematic phase diagram was established for this PEO-KOH-H2O system. The first conductivity measurements also revealed the great interest of these systems, with conductivity values up to 10(-3) S/cm.