In summary, we developed an all-optical approach to quantify the interface energetics of ML-MoS2 semiconductor-electrolyte interfaces. The method involves (1) measuring potentialdependent absorbance spectra; (2) fitting the neutral exciton and trion features in the absorbance spectrum using the MND model to extract the carrier concentration 𝑛; (3) interpolating the electronic band gap at each 𝐸 using 𝑛; (4) constructing an energy level diagram assuming a known, potential-independent CB edge value. Our results show that band edge movement in ML-MoS2 is significant (0.2-0.5 eV) over a narrow range of applied potentials (0.2-0.3 V). The band edge movement magnitude depends on the calculated 𝑛 values. Large band edge shifts have important consequences for practical electrochemical applications involving interfacial charge transfer because kET could change by a factor of 10-100. Future experimental and theoretical work is needed to accurately measure 𝑛 from in situ spectroelectrochemical measurements such as absorbance/reflectance, Raman, or PL spectroscopy.