The charge-transfer quenching of photoexcited TiO2 nanoparticles by hexanethiolate monolayer protected gold clusters (MPCs) with core diameters of 1.1-4.9 nm is reported. The quenching efficiency of Au MPCs was compared using Stern-Volmer plots. The quenching constants (K-Q) taken from slopes of the plots reveal that K-Q increases from 5.6 x 10(5) to 4.09 x 10(7) by more than 70-fold as the core diameter increases from 1.1 to 4.9 nm. The origin of the size dependence is considered. K-Q was found to linearly correlate with the MPC capacitance. This suggests that the quenching process is dominantly controlled by the capacitance of the electron acceptor (Au MPC). K-Q for 1.1 nm diameter Au MPC was found to be significantly smaller than that predicted by the K-Q-capacitance correlation. This may reflect the size-dependent opening of an energy gap at the Fermi level for molecule-like MPCs.