Concentration dependences of a complete set of ionic transport properties are reported for the binary solid polymeric ion conductor poly(ethylene-oxide) [P(EO)], complexed with NaTFSI, [TFSI = (bis)trifluoromethanesulfonate imide, (CF3SO2)(2)N]. Measured transport properties include the ionic conductivity to), the differential salt diffusion coefficient (D-s), and the cationic transference number (t(+)(0)). To measure t(+)(0), a recently developed technique based on concentrated solution theory is used. Under this framework no assumptions need to be made concerning Ideality of the electrolytic solution. We find that for the P(EO)(pi)-NaTFSI complexes the ionic conductivities, and differential salt diffusion coefficients are much higher than those of the previously reported P(EO)(pi)-NaCF3SO3 system. D-s also varies less with complex composition in concentrated solutions. Interestingly, the cationic transference numbers are found to be negative over a wide concentration range, i.e., t(+)(0) decreases with increasing salt concentration and reaches a low of -1.15 +/- 0.25 at a composition corresponding to an ether-oxygen to sodium-ion ratio of 7:1. This implies that the cationic current is mainly carried by complexed ions. Complementary Raman spectroscopic studies directly probing cation-anion and cation-polymer molecular interactions show that extensive interionic interactions occur in this system.