Solar hydrogen production with semiconductor photocatalyst particles typically requires co-catalysts to accelerate the hydrogen evolution reaction, but since co-catalysts are often deposited in situ, the rate of their nucleation/growth and role in parasitic light absorption are not well controlled. Herein we introduce a halted photodeposition-dialysis method that affords unprecedented control over platinum (Pt) co-catalyst loading and morphology on bulk heterojunction organic semiconductor photocatalyst nanoparticles. Pt loading and surface distribution are controlled by tuning the initial Pt precursor concentration and photodeposition time followed by removal of unreacted Pt precursor via dialysis. Applying this method with typical Pt deposition conditions gave a max H 2 evolution rate of 140 mmol h −1 g −1 (based on semiconductor mass) with only 15.2 wt % Pt as measured by ICP-MS and suggested an optimum loading of < 20 wt % Pt, above which parasitic light absorption decreases the H 2 evolution rate. Moreover, a peak H 2 evolution >30 mmol h −1 g −1 is achieved with a Pt loading of only 1.01 wt % by tuning the deposition conditions to favor a more uniform Pt coverage with small clusters and single atoms over larger Pt NPs. Representing a performance more than 8 times higher compared to typical Pt photodeposition (based on Pt) and giving critical insights into optimizing performance for solar driven H 2 production.