Gallium phosphide (GaP) is a semiconductor with conduction and valence bands that straddle the potential of the proton reduction half-reaction and a bandgap of 2.26 eV, allowing for absorption of a significant portion of the solar spectrum and generation of a large photovoltage. These properties give GaP great potential as a photocathode material for the solar-driven water splitting reaction. However, a hurdle that needs to be overcome for the implementation of this material in photoelectrochemical cells is the large overpotential for the proton reduction half-reaction. This study investigates the pairing of cobalt bis(dichlorobenzenedithiolate) ((TBA)[Co(Cl₂bdt)₂]) – a molecular HER catalyst – with GaP photocathodes. (TBA)[Co(Cl₂bdt)₂] demonstrates good catalytic activity for proton reduction in acidic media and has previously been shown to strongly physisorb to graphitic surfaces through π-interactions. Herein is presented investigations into the physisorption of (TBA)[Co(Cl₂bdt)₂] to single crystal GaP wafers through a transparent, conductive graphitic thin film. Reduced graphene oxide (RGO) thin films are prepared on GaP through a novel method that is exceedingly gentle towards the underlying semiconducting material. Because of their graphitic character, these RGO films can participate in the π-interactions necessary for catalyst adsorption. X-ray photoelectron spectroscopy and electrochemistry are used to detect the presence of the catalyst on the RGO films. Photoelectrochemical experiments have been performed investigating the effects of an RGO thin film on homogeneous light-driven electrocatalysis at GaP photocathodes. Further experiments will delve into heterogeneous catalysis in which the catalyst is physisorbed to the GaP/RGO photocathode. Additionally, finite-element models of catalyzed photoelectrochemical reactions at a GaP-electrolyte interface will be discussed.