This study, performed on mixtures of NaClO4 and NaCF3SO3 with atactic poly(propylene oxide) (PPO), confirms that a microphase separation phenomenon takes place in both low and high molecular weight PPO. Below a certain concentration, which corresponds to O/Na = 13 (O = ether oxygen) in a M = 4 x 10(3) PPO and to O/Na = 11 in a M = 2 x 10(5) PPO, optically clear mixtures exhibit two glass transition (T-g) features indicating that a salt-rich microphase separates from the polymer. A comparison with poly(ethylene oxide) (PEG) amorphous mixtures containing the same salts shows that this phenomenon has a dramatic effect on ion conduction. It also reveals that an ion-entrapping structure, similar to that in the salt-rich microphase, is present over a range of concentrations above the critical composition. Although chain length has a marginal effect on conductivity in PEG, it has a strong effect on conductivity in PPO. As evidenced by a sharper T-g splitting in the high molecular weight PPO, this effect is due to the formation of larger microdomains. From this feature and the change in the microdomain composition, it may be argued that solvating power of PPO decreases with increasing chain length or decreasing OH end-group density.