A DSC study performed on poly(propylene oxide) (PPO) electrolytes containing various lithium salts (LiBr, LiClO4, LiCF3SO3, and LiN(CF3SO2)(2)) shows that microphase separation is a general feature of PPO-LiX systems. Below a certain salt content, which corresponds to O/Li = 9 (O = ether oxygen) for LiBr, to O/Li = 10 for LiClO4 and LiCF3SO3, and to O/Li = 16 for LiN(CF3SO2)(2), two glass transition (T-g) features are recorded on optically clear mixtures of these systems. A comparison made with poly(ethylene oxide) (PEG) electrolytes shows that this phase complication has a strong effect on the conduction process. For the PPO-LiBr and PPO-LiClO4 systems, which involve a large difference between the compositions of their low-T-g and high-T-g microphases, a percolation threshold occurs over the range where related PEO-LiX electrolytes exhibit their conductivity maximum. A similar but less pronounced effect is systematically observed for the other PPO-LiX systems and for non-PPO systems that exhibit an initial accelerated rise in their T-g-composition relationships. The PEO-LiX systems are free from these anomalies.