A commonly followed strategy to enhance the performance of photoelectrodes for achieving viable water splitting devices consists in the use of co-catalysts. However, fine control of the amount of co-catalyst deposited on the electrode surface usually does not receive much attention, although it is an essential factor that crucially determines the efficiency of photoelectrodes. In this context, this study illustrates how the dark electrochemical characterization of co-catalyst-modified photoanodes may provide valuable information on the precise amount of electroactive co-catalyst present on the surface, facilitating our understanding of the manner in which co-catalysts work and paving the way for their optimization. For this purpose, ultrathin Ni(OH)(2) layers were deposited on either doped or pristine BiVO4 and FeVO4 photoanodes by a cost-effective and versatile chemical bath deposition method in which the deposition time allows to control the Ni(OH)(2) loading. The deposited Ni(OH)(2) is demonstrated to successfully catalyze the photoelectrochemical water oxidation process on both BiVO4 and FeVO4 electrodes, by improving the effective transfer of photogenerated holes from the semiconductor to solution. In fact, the electrocatalytic activity of the Ni(OH)(2)-modified photoanodes rapidly increases with the amount of deposited Ni(OH)(2) until an optimum value is reached equivalent to only 0.46 and 1.68 Ni(OH)(2) monolayers for BiVO4 and FeVO4 electrodes, respectively. Under these conditions, Ni(OH)(2) enhances the photocurrent generation by a factor of 2 for BiVO4 and of 1.2 for FeVO4 photoanodes in contact with slightly alkaline solutions. The small loadings of Ni(OH)(2) leading to an optimum photoanode behavior indicate that hole transfer is favored on vanadate active surface sites, which would also be the preferential sites for Ni(OH)(2) deposition. Additional amounts of co-catalyst deposited for longer times induce a drastic decrease in the photoelectrocatalytic activity due to charge recombination at the semiconductor/Ni(OH)(2) interface, probably enhanced by the low carrier mobility in Ni(OH)(2). On the other hand, this study demonstrates that the preparation of efficient photoanodes for oxygen evolution with an extremely small amount of co-catalyst through a facile and scalable method is possible. (C) 2017 Elsevier B.V. All rights reserved.