AbstractThe future of mobility depends on the development of next‐generation battery technologies, such as all‐solid‐state batteries. As the ionic conductivity of solid Li⁺‐conductors can, in some cases, approach that of liquid electrolytes, a significant remaining barrier faced by solid‐state electrolytes (SSEs) is the interface formed at the anode and cathode materials, with chemical instability and physical resistances arising. The physical properties of space charge layers (SCLs), a widely discussed phenomenon in SSEs, are still unclear. In this work, spectroscopic ellipsometry is used to characterize the accumulation and depletion layers. An optical model is developed to quantify their thicknesses and corresponding concentration changes. It is shown that the Li⁺‐depleted layer (≈190 nm at 1 V) is thinner than the accumulation layer (≈320 nm at 1 V) in a glassy lithium‐ion‐conducting glass ceramic electrolyte (a trademark of Ohara Corporation). The in situ approach combining electrochemistry and optics resolves the ambiguities around SCL formation. It opens up a wide field of optical measurements on SSEs, allowing various experimental studies in the future.