In this study, the salt concentration in a concentrated binary polymer electrolyte was measured in situ by means of confocal Raman spectroscopy during galvanostatic polarization experiments. The electrolyte studied was 0.8 M lithium bis(trifluoromethanesulfone)imide in a copolymer of ethylene-and propylene oxide at 25degreesC. Recent work with a transport model and characterization of the transport properties, for the same electrolyte, was verified with the spectroscopic results of this study. A good agreement between modeled and measured results was found. The spectroscopic method suited well for these studies. The possibilities of using a transport model are briefly demonstrated and discussed.