Understanding the cocatalyst/semiconductor interaction is of key importance for the design and synthesis of next generation photocatalytic materials for efficient hydrogen production and environmental cleanup applications. Here we investigate preformed Pd nanoparticles (NPs) supported on a series of anatase TiO ₂ having well-controlled but varying degrees of crystallinity and crystallite size, and explore their photocatalytic performance for H ₂ production and phenol decomposition. While tuning the anatase crystallite size significantly influences the photocatalytic performance, varying the TiO ₂ crystallinity shows a negligible effect. Interestingly, the optimum quantum efficiency (∼78%) for H ₂ evolution is achieved with anatase having medium crystallite size (∼16 nm), whereas for phenol decomposition, a promotional effect is only observed for anatase with larger crystallite sizes (>20 nm). Surface radical species and radical densities study reveal that the photogenerated charge carriers have been trapped at different sites depending on the crystallite size of anatase. While the excited electrons are only trapped in bulk lattice sites in small anatase (<16 nm), larger anatase particles provide extra surface sites for charge trapping, which benefit charge storage and transportation to Pd surface sites, leading to a more efficient utilization of charge carriers for photocatalysis. Additionally, Pd supported on medium sized anatase (∼16 nm) hinders the formation of O ₂ ^•- radicals on TiO ₂ surfaces, thus preventing unwanted reoxidation of photogenerated H ₂.