Photoelectrochemically (PEC) driven reactions are an attractive approach to the capture and storage of solar energy. The quest for efficient and robust solar fuel formation motivates the development of increasingly complex semiconductor surface architectures with correspondingly intricate interfacial energetics, such as oxide deposition, catalyst incorporation, or surface texturing. PEC performance is a function of kinetic and thermodynamic aspects at the semiconductor/liquid junction, and deconvoluting these elements is therefore critical to understanding and improving interfacial dynamics. The flatband potential is one measure of thermodynamics at the semiconductor surface and – when compared with voltammetric data – also allows for an indirect assessment of interfacial kinetics. While electrochemical impedance spectroscopy (EIS) and subsequent Mott–Schottky analysis is the historical standard for flatband potential measurements, additional capacitive elements from surface oxides, catalysts, or other deviations from ideal semiconductor/liquid junctions complicate this analysis. Accordingly, we developed the Intensity-Modulated High-Frequency Resistivity (IMHFR) apparatus to study dynamic and complex semiconductor/liquid junctions. In this method we use a large AC frequency (100 kHz) to measure the space–charge resistance (RSC ) of a semiconductor/liquid junction under chopped light as a function of potential. We then isolate potential regimes where RSC is dependent and independent of illumination and extract the potential at the boundary between the two regimes, which we define as the flatband potential. We apply this technique to nanoporous ‘black’ silicon in order to demonstrate its utility and characterize the thermodynamic and kinetic effects of surface treatments at a complex semiconductor/liquid junction. Black silicon is an exciting improvement upon traditional silicon for its large surface area and increased light absorption. We form organic/inorganic films– known to favorably shift proton reduction onset potentials – on black silicon by first covalently binding organic monolayers to nucleate inorganic film growth by atomic layer deposition (ALD). After depositing TiO2 and Pt we collected voltammograms and IMHFR data in acidic media to characterize proton reduction behavior. We find that while the flatband potential shifts positively (favorable for proton reduction), interfacial kinetics are limiting as thicker TiO2 layers are deposited. To circumvent the kinetic limitation, we bury Pt nanoparticles directly into the silicon and form the organic/inorganic film around the nanoparticle. We find the direct Si/Pt contact improves interfacial kinetics but at the cost of the positive shifts to the flatband potential we observed previously. This analysis on a textured and layered semiconductor interface demonstrates the analytical power of the IMHFR technique.