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Conductivities ( cr), salt diffusion coefficients (Ds), and cationic transference numbers ( t~) are reported for a high molecular weight polypropylene oxide (Parel™)-LiCF3S03 polymer electrolyte system at 85° C. Transference numbers were determined as a function of salt concentration using a recently described electrochemical method based on concentrated solution theory. For the Parel-LiCF 3 S03 system, t~ is slightly positive for electrolytes with O:Li ratios of 15 or 12:1, but decreases to negative values for more concentrated solutions. This implies that negatively charged ionic aggregates such as triplets are more mobile than free cations in this concentration range. Such behavior is commonly seen in binary salt/polymer electrolytes, which typically exhibit a high degree of non-ideality. The non-unity transference numbers and microphase separation in the Parel-LiCF3S03 system strongly suggest that salt precipitation or phase separation will occur in operating cells containing these electrolytes, due to the development of large concentration gradients during passage of current.