The processes occurring in the PAN-based gel polymer electrolyte have been in situ investigated by micro-Raman spectroscopy. It is found that in the polarized membrane a stable gradient of both Li cations and perchlorate anions appears: at the anode side of the assembly, the concentration of cations is essentially higher after polarization, while at the cathode side it is visibly lower; in the middle of the membrane concentration of charge carriers remains practically unchanged. While observed gradients are expected in a working electrolyte, an unexpected, and possibly more serious, depletion of the CN groups of the PAN matrix is observed at the anode side during polarization. The nature of this effect may be ascribed to the polymer mobility due to the strong polymer–electrolyte interaction and/or to the polymer degradation caused by internal heating under applied potential.