Energy and electron transfer processes between photo-excited semiconductor and metal are important phenomena in photocatalysts. While metals in contact with semiconductor nanoparticles have been employed to enhance the overall photocatalytic activity, the interfacial electron and energy transfer processes are yet to be understood fully. We present excited-state deactivation studies of photoactive semiconductor nanoparticles, such as TiO2 and CdSe quantum dots (QDs), by structurally and energetically well-defined alkanethiolate-coated monolayer-protected gold clusters (MPCs) with core size ranging 1-5 nm. The charge transfer efficiency from a photo-excited TiO2 to MPCs was monitored by the decrease in the blue coloration due to photogenerated electrons. The charge transfer efficiency significantly increased as increasing MPC core size from 1 to 5 nm, revealing the effect of the charging capacitance of MPCs. The MPC core-size dependent deactivation process was also found in a CdSe QD-MPC system. The emission spectra of CdSe QDs were collected as a function of MPC concentration in the mixture of the QD and MPC colloidal solution. The resulting Stern-Volmer quenching constants were found to increase considerably as increasing the MPC core-size from 1 to 5 nm.