The experimental behaviour of photoelectrochemical materials illuminated under high irradiance conditions >100 kW m−2 has not been studied despite being potentially advantageous for improving the photoelectrochemical performance and the system-level design through the miniaturisation of PEC cells, and for providing conditions that can mimic accelerated ageing or long term operation. This study presents the design of a high flux photoelectrochemical (HFPEC) test cell, which ensured adequate cooling through forced convention, and experimental setup in a high flux solar simulator for the study of light-dependent behaviour of two reference materials, Sn-doped Fe2O3 and BiVO4, under high irradiances (up to 358 kW m−2). Current densities of up to 1500 and 300 A m−2 were achieved for FTO|Fe2O3 and FTO|BiVO4, respectively. To qualitatively deconvolve the different phenomena and their effects, temperature dependence studies under approx. 1 sun (1 sun = 1 kW m−2) illumination were performed. It was found that the sublinear light-dependent behaviour was not explained by the temperature increase under illumination and, based on multiphysics modelling, likely primarily arises from bubble-induced losses. Furthermore, whilst the overall degradation rate increases for FTO|BiVO4 samples under increasing irradiance, a fitted phenomenological model indicates that the degradation kinetics are light-dependent, where increased irradiance diminishes the fraction of charge consumed by photocorrosion reactions. This study highlights the potential of HFPEC experiments to contribute to the scientific analysis of semiconductor–electrolyte behaviour at high photon flux conditions and to identify and resolve the practical challenges of engineering HFPEC devices.