Platinum cocatalysts are essential for photocatalytic hydrogen evolution by organic semiconductor (OSC) bulk-heterojunction nanoparticles (BHJ NPs), yet the surface chemistry of photodeposited Pt remains poorly understood. Here, we show that the halide ligand in hexahaloplatinate(IV) precursors (K2PtX6, X = Cl, Br, I) critically governs both the kinetics of Pt photodeposition and the resulting photocatalytic performance. Chloroplatinate precursors leave partially reduced [PtClx]n species adsorbed on the Pt surface, poisoning active sites and severely suppressing H2 evolution rates in BHJ systems that generate low photopotential, which is confirmed by photoelectrochemical measurements and Kelvin-probe force microscopy. Bromo- and iodoplatinate precursors reduce more readily, largely avoiding this poisoning. Remarkably, the addition of iodide ions to the photocatalytic mixture provides a substantial and general boost to H2 evolution rates, reaching an apparent quantum yield of 17% at 700 nm (among the highest reported for BHJ NP photocatalysts) attributed to beneficial iodide adsorption on the Pt surface.