New alkali metal salts capable of forming ionic complexes with poly(ethylene oxide) (PEG) have been prepared from the amide CF3SO2NH(CH2)(3)OCH3. Among these salts, the potassium salt KCF3SO2N(CH2)(3)OCH3 (KMPSA), which has both a low melting point (T-m = 45 degrees C) and a low glass transition temperature (T-g = -9 degrees C), is miscible in all proportions with PEO at moderate temperatures. This allowed, for the first time, a study of the T-g-composition and conductivity-composition relationships of PEO rubbery electrolytes from the semidilute regime to the molten salt. A comparison with LiCF3SO2N(CH2)(3)OCH3 (LiMPSA), on the one hand, and with Li(CF3SO2)(2)N (LiTFSI) and K(CF3SO2)2N (KTFSI), on the other hand, shows other features resulting from the presence of the methoxypropyl group in the new anion. At moderate salt contents (EO/salt > 8), the T-g elevation produced by LiMPSA (1.3 degrees C/mol %) is substantially lower than that produced by KMPSA (2.3 degrees C/mol %) or LiTFSI and KTFSI (2.8 degrees C/mol %). Over this range, cation charge density has about no effect on the reduced (T - T-g = constant) molar conductivities of LiTFSI and KTFSI, while it has a strong effect on those of LlMPSA and KMPSA. The relationship obtained from KMPSA amd L iTFSI, which extend to the bulk salt and to EO/salt = 2, respectively, show a change of regime that may be interpreted in terms of a percolation threshold. This feature, which is caused by the depletion of the free EO units, is evidenced by a sigmoidal decrease in the reduced molar conductivity. For EO/salt < 3, the rubbery electrolytes consist of fully complexed PEO dissolved in molten KMPSA or molten LiTFSI (T-g = 50 degrees C).