A systematic investigation on the photoelectrocatalytic (PEC) performance of a series of CuW1-xMoxO4 materials with different Mo for W substitution (x = 0-0.8), successfully synthesized as single, transparent photoactive layers, allowed us to identify copper molybdo-tungstate with x = 0.5 (CuW0.5Mo0.5O4) as the best performing Mo-containing CuWO4-based material for photoanodes fabrication. For 250 nm thick material, the CuW0.5Mo0.5O4 exhibits a 6-fold photocurrent increase at 1.23 V vs. RHE with respect to pure CuWO4. Both PEC analyses in the presence of NaNO2 as sacrificial agent and intensity modulated photocurrent spectroscopy (IMPS) measurements, here applied to this class of materials for the first time, demonstrate that the superior PEC performance of CuW0.5Mo0.5O4 stems from a more efficient separation of photoproduced charges with respect to CuWO4, while the charge injection efficiency is close to 100% for both materials. Further enhanced separation of photoproduced charges, resulting in increased PEC performance of the CuW0.5Mo0.5O4 electrode in the 400-480 nm wavelength range, can be achieved by coupling it with BiVO4, to form a type II heterojunction system.