Developing high-performance photoelectrodes for outdoor operation of (photo)electrochemical solar fuel devices requires elucidating the stability of photoelectrodes under diurnal operating conditions. In this work, we establish an experimental approach to understand how the dynamic microenvironment associated with diurnal cycling impacts durability, utilizing a model system consisting of Si coated with a thin-film layer of Mo/MoS2 to serve as a “protective” layer and catalyst to drive the H2 evolution reaction. Our efforts combine electrochemical methods with online inductively coupled plasma mass spectrometry, operando electrochemical mass spectrometry, and Raman spectroscopy to identify degradation mechanisms under simulated diurnal cycling. Our results show that under simulated daytime conditions, S slowly leaches from the MoS2 surface as H2S. At “nighttime,” simulated by open-circuit conditions, we observe Mo dissolution, which is further accelerated when exposed to increasing concentrations of dissolved O2. These insights enable a design strategy to mitigate corrosion, demonstrating that small expenditures of power during nighttime conditions can suppress O2-driven Mo dissolution. This work presents an analysis of diurnal irradiation-driven (photo)electrode degradation that can be used to engineer resilience in future (photo)electrochemical systems.