Our experimental proof that the maximum achievable quasi-Fermi level of electrons is less negative after Mo-doping is significant and deserves a more detailed discussion. In this context, we point out that it is well-established that the maximum achievable photovoltage in metal oxide photoanodes is typically limited by the presence of intragap electron polaronic states formed upon self-trapping of excess electrons at reduced ionic sites (e.g., Fe 2+ in hematite Fe2O3, or V 4+ in BiVO4) in the lattice, accompanied by the displacement of surrounding atoms via electron-phonon interactions, forming thus a quasi-particle called polaron. As the electron polaron level is typically significantly below the conduction band edge, it effectively traps electrons, limits their mobility, acts as a recombination center, and therefore establishes an inherent limitation of the maximum achievable quasi-Fermi level of electrons and photovoltage. For both pristine and doped BiVO4, it is reported that the excess electrons are localized in hybridized vanadium which is accompanied by elongation of the V-O bond by ~0.1 Å and formation of small polarons located ca. 0.88 eV below the conduction band edge. Doping by molybdenum substitutes V 5+ with ionized Mo 6+ dopants, whereby the excess electron is readily localized at V 4+ polaronic states. The detrimental polaronic effects can be then (at least partially) avoided by applying a sufficient external electric bias, which, however, compromises the overall photoconversion efficiency. To summarize, the conductivity enhancement upon Mo-doping, which is apparent from enhanced photocurrents at higher bias potentials, is mainly due to the increased electron concentration, whereby the electron mobility is apparently not improved. However, at low bias potentials the beneficial effect of the Mo-doping is counterbalanced by enhanced recombination due to the presence of deep electron polaron states, which makes itself apparent by decreased photocurrents at low bias potentials and by a less negative OCP (i.e., less negative quasi-Fermi level of electrons) under irradiation.