Gold nanoparticles located at a metal oxide/hole conductor interface generate photocurrents upon visible light illumination. We demonstrate that the quantum efficiency of this process depends on the nanoparticle size. Gold nanoparticles (5 nm) show a maximum absorbed photon-to-electron conversion efficiency (APCE) of 13.3%. For increasing particle sizes, the average APCE decreases to 3.3% for the largest particles (40 nm) investigated. Three possible causes for this efficiency change are discussed.