The complete phase diagram of the poly(ethylene oxide)/lithium trifluoromethane sulfonate polymer electrolyte was determined by combined X-ray diffraction and differential scanning calorimetry. The sample preparation has dramatic effects on morphology and consequently on the phase diagram. The existence of three different molecular adducts involving the polymer and the salt was deduced from the phase diagram. These adducts transform into each other by peritectic reactions. Moreover, two different forms of pure lithium trifluoromethane sulfonate were observed. This polymorphism was studied by X-ray diffraction and differential scanning calorimetry. It was demonstrated that this salt undergoes a transition to a phase of higher symmetry at 179 degrees C. An orthorhombic unit cell (a = 1.496 nm, b = 1.475 nm, c = 1.425 nm) was proposed for the high temperature form of the salt.