The influence of an earth-abundant water oxidation electrocatalyst (Co-Pi) on solar water oxidation by W:BiVO4 has been studied using photoelectrochemical (PEC) techniques. Modification of W:BiVO4 photoanode surfaces with Co-Pi has yielded a very large (similar to 440 mV) cathodic shift in the onset potential for sustained PEC water oxidation at pH 8. PEC experiments with H2O2 as a surrogate substrate have revealed that interfacing Co-Pi with these W:BiVO4 photoanodes almost completely eliminates losses due to surface electron-hole recombination. The results obtained for W:BiVO4 are compared with those reported recently for Co-Pi/alpha-Fe2O3 photoanodes. The low absolute onset potential of similar to 310 mV vs RHE achieved with the Co-Pi/W:BiVO4 combination is promising for overall solar water splitting in low-cost tandem PEC cells, and is encouraging for application of this surface modification strategy to other candidate photoanodes.