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Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕ CS ) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕ abs ), charge separation (ϕ CS ), and photoreduction (ϕ red ), i.e ., IPCE = ϕ abs ϕ CS ϕ red . Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕ CS and interfacial recombination at the junction that decreases ϕ red . Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO 2 , the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.