Conductivity measurements in PEO30MI polymer electrolytes with M = Li, Na, K, Rb, or Cs over the temperature range from about 65 to 200 C show an increasing tendency for salt precipitation with increasing cation size. The salt precipitation in these complexes upon heating is revealed by the decrease of the do conductivity starting at a critical temperature T-c. Whereas LiI and NaI complexes do not show precipitation effects. T-c monotonically decreases from about 140 to 65 degrees C when changing the salt component from KI via RbI to CsI. For the PEO-RbI system, precipitation is further investigated by nuclear magnetic resonance (NMR) and tracer diffusion experiments. NMR analysis unambiguously demonstrates the onset of RbI salt precipitation and the increase of the precipitate fraction with increasing temperature. In diffusion experiments on PEO30RbI with the radiotracers Rb-86 and I-125, the precipitation effect is manifested by anomalous features in the penetration profiles, however, without noticeable changes in their depth range. Combining the resulting tracer diffusion coefficients with the do conductivity data enables us to assess crucial parameters characterizing ionic transport in PEO30RbI. (C) 2009 Elsevier Ltd. All rights reserved.