The photoluminescence quenching of a CdSe quantum dot by hexanethiolate-monolayer-protected gold clusters(MPCs) with core diameters of 1.1-4.9 nm is described. Experimental evidence suggests that the quenchingoccurs via an energy transfer mechanism. The energy transfer quenching efficiencies of MPCs were comparedusing Stern-Volmer plots. The quenching constant (KQ) obtained from the slope of the plot shows an enormousincrease from 2.5 × 105 to 2.3 × 108 M-1 by nearly 1000-fold as the core size increases from 1.1 to 4.9 nm.The origin of the size dependence is considered. There is a remarkable linear correlation found between thequenching constant and the MPC core volume. This correlation suggests that the energy transfer quenchingefficiency is governed by the absorption cross-section of the MPC quencher that scales as its core volume.