High molecular weight poly(ethylene oxide) (PEO) and a polymer electrolyte based on PEO containing the divalent metal halide salt, NiBr₂, have been characterized using simultaneous thermomechanical and dielectric thermal analysis. Simultaneous measurement on the same sample allows separation of effects caused by simple expansion or flow from those caused by chemical reaction or phase transition. In fresh “as cast” samples, particularly under low load conditions, unexpectedly large expansions have been observed below room temperature on passing through the glass transition, and contractions have been seen on melting of pure PEO phases in both systems. Normal behavior, however, is observed after a sample has been melted and allowed to cool in the TMA. In NiBr₂PEO₈, a very large transitory expansion, up to 50%, has been observed well above the crystalline PEO melting temperature that appears to correlate with precipitation of the metal salt from solution.