Using a scanning tunneling microscope, elect ron-stimulated photon emission spectra have been measured for individual Ag clusters on differently pretreated TiO2(1 1 0) substrates. For clusters on weakly reduced TiO2, we observe radiating decays of Mie-plasmons shifting to higher energies as cluster sizes decrease. On strongly reduced TiO2, plasmons are dissipated via energy transfer to electron-hole pair excitations in the oxide and emission spectra reveal radiating decays of TiO2 excitons.