Water oxidation photoanodes based on earth-abundant metal oxide semiconductors are actively studied due to their limited environmental impact, the stability to photo-corrosion and the tunable optoelectronic properties [1]. BiVO₄/WO₃ heterostructures are now emerging as one of the most promising photoanode, but the activity is limited by surface recombination and sluggish charge transfer kinetics. Cobalt-iron mixed compounds, such as CoFeOx and CoFe hexacyanoferrate, a Prussian blue analogue, proved to effectively enhance photocurrent when coupled to BiVO₄/WO₃ photoanodes as water oxidation co-catalyst. The origin of the increased efficiency is still debated, but recent studies suggest an enhancement of charge separation efficiency within the semiconductor [2], rather than improved water oxidation kinetics, underlying possible charge accumulation on the co-catalyst leading to transient modification of the local structure. X-ray Absorption Spectroscopy (XAS) provides a unique view of the structural features of ultrathin catalyst layers, however often limited to a steady state ex-situ characterization of the catalyst local structure and valence state. To achieve structural understanding of the catalyst activity in the actual PEC operating conditions, a custom PEC-XAS cell was specifically designed to allow for operando monitoring of Co-Fe based co-catalysts deposited on  BiVO₄/WO₃ photoanodes, upon bias and illumination stimuli fully replicating the ex-situ conditions. The developed setup (Fig 1a), providing limited x-ray photons interaction path with the controlled-flow electrolyte (<100 μ m), prevents bubbles formation and enhances mass transfer (Fig 1a), allowing for stable, real-time correlation of the structural features with the photoelectrochemical response. The activity of CoFeOx and CoFe-PB  was investigated by either performing fixed potential spectroscopy (Fig 1b) at Co K-edge, or fixed X-ray energy absorption voltammograms [3] (FEXRAV, Fig 1c). The resulting characterization displayed a drastic modification of the structure between operando and ex-situ analysis, as well as a dependence on bias and illumination. In addition, despite the apparent similarities between CoFeOx and CoFe-PB in terms of composition and PEC activity, their operando-XAS behaviour displays a different enhancement mechanism, further correlated to the charge dynamics characteristics, investigated by IMPS and PEIS.