Rate constants for electron-transfer quenching of the lowest excited state of Pt(Ph2phen)(ecda), where Ph2phen is 4,7-diphenyl-1,10-phenanthroline and ecda is 1-(ethoxycarbonyl)-l-cyanoethylene-2,2-dithiolate, by a series of neutral amine quenchers have been determined. Also determined are the electron-transfer and energy-transfer quenching rates of the excited complex *Pt(Ph2phen)(ecda) by a series of metallocene quenchers. The triplet states of the amine quenchers are relatively high in energy, >62 kcal/mol, compared to the triplet state of Pt(Ph2phen)(ecda) at 52 kcal/mol so that electron transfer is the only possible pathway. However, the low triplet-state energies of the metallocene quenchers, 38-46 kcal/ mol, permit energy transfer to compete with electron transfer in this system. Marcus analysis of the amine quenchers allows determination of the excited-state reduction potential, E(Pt*/-), of Pt(Ph2phen)(ecda) at 0.93 V. E(Pt*/-) is also estimated using the ground-state reduction potential of Pt(Ph2-phen)(ecda) of -1.24 V vs SCE and the crossing point of the emission and excitation spectra of the complex at 2.22 V. This method of estimation yields a value of 0.98 V for E(Pt*/-) in agreement with the value obtained from the quenching studies. The correlation of E(Pt*/-) through the two methods allows estimation of E(Pt*/-) of related Pt(diimine)(dithiolate) complexes.