Abstract Understanding the electrochemical and mechanical behavior of solid electrolytes beyond their electrochemical stability window is crucial for enabling high energy density all‐solid‐state batteries. Accordingly, this work systematically studies a model working electrode of Li 3 PS 4 , ball milled with vapor grown carbon fiber (VGCF). Operando X‐ray photoelectron spectroscopy can identify and quantify the potential‐dependent redox byproducts, their reversibility, and electrical properties, while operando cell pressure measurements correlate these with volume changes and mechanical instability. The study examines voltages up to 5.0 V and down to −0.05 V versus Li/Li + , mimicking cathode and anode cycling. It demonstrates that within the 2.4–5.0 V region, Li 3 PS 4 oxidation byproducts are primarily polysulfides composed of bridging sulfurs (P‐S‐S‐P) between PS 4 3‐ units, free of elemental sulfur (S 0 ), and electrically conductive. The Li 3 PS 4 oxidation process occurs at 2.8 V during first charge and ends at 3.4 V, with volume shrinkage at the VGCF interface. During reduction (2.4 to −0.05 V), polysulfides convert reversibly to Li 3 PS 4 between 1.9 and 1.7 V, then to Li 2 S and Li n P (0 ≤ n ≤ 3) between 1.9 and 0.6 V, causing volume expansion and the transition to an electrically insulating interphase. Below 0.6 V, Li 2 O formation dominates without further evolution of Li 2 S or Li n P.