A combined experimental-numerical approach was used to study transient phenomena occurring in a photoelectrochemical cell using a membrane-separated porous TiO2-based photoanode and a dark Pt-based cathode. The effects of three parameters (pH in the anodic compartment, operating cell temperature, and cathode compartment preconditioning with hydrogen) on the photocurrent was systematically investigated using design of experiments and analysis of variance. A theoretical model was developed able to accurately reproduce and predict the measurements. The model indicated that the electrochemical reaction uses two parallel pathways on the anodic interface. The first pathway represented the rapid charging of surface states and the subsequent formation of acidic titanol groups at the TiO2/H2O interface which, upon illumination, caused an anodic overshoot at a short timescale. These states recombined with the formed O2 at a longer timescale which resulted in a current decrease after the overshoot. The second pathway was governed by transfer processes of H+ ions at the TiO2/Nafion® interface and caused the observed current increase under illumination and positive relaxation in the dark, both at long timescales. A negative undershoot was observed when the reverse electrolysis reaction was preferred.