The electrochemical stability window of NASICON-structured glass-ceramics of the Li1+xCrx(GeyTi1-y)2-x(PO4)3 system is investigated by a combination of cyclic voltammetry, electrochemical impedance, and X-ray photoelectron spectroscopy techniques. Cyclic voltammetry analyses are performed using a three-electrode setup cell where Ag3SI/Ag is applied as a reference electrode. Cyclic voltammetry measurements are followed by in situ electrochemical impedance spectroscopy, enabling the effect of oxidation and reduction reactions on the electrical properties of the glass-ceramics in question to be determined. X-ray photoelectron spectroscopy, in turn, is applied to determine which chemical species undergo reduction/oxidation. Our findings reveal that the electrochemical stability of this material is limited by the reduction of Ti+4 cations in low potentials (around 2.1 V vs. Li+/Li) and by the oxidation of O−2 anions in high potentials (about 4.8 V vs. Li+/Li). After the first cycle, the electrolytes seem to be stable within a much broader electrochemical window than in the first cycle. However, the results indicate not only that the reduction at low potential is not deleterious to the electrical properties of the electrolytes but also that the oxidation reaction at high potentials is highly detrimental. These findings contradict the common perception about the outstanding stability of NASICON-structured electrolytes in oxidation potentials.