The binding constant of Ru(phen)(3)(2+) to starburst dendrimers (SBDs) was determined utilizing changes in the probe’s excited state lifetime as a function of SBD concentration. The increase of the probe’s lifetime in the presence of SBDs has been attributed to a lower O-2 concentration at the dendrimer’s surface than in water, which acts as a quencher of the triplet metal-to-ligand charge transfer (MLCT) excited state. The current study was performed at high SBD concentrations, relative to those of the probe and quencher, and in this concentration range the late of quenching of *Ru(phen)(3)(2+) by Co(phen)(3)(3+) bound to SBDs is independent of quencher concentration. These results conclusively indicate that the quenching is intrastarburst in nature. Furthermore, in this high SBD concentration limit the quenching rate constants are obtained with a simple model and without extensive numerical fitting of the luminescence decay.