The acid-base behavior of the ground and excited states of Pt(qdt)(2)(2-) and Pt(phen)(qdt), where qdt = quinoxaline-2,3-dithiolate and phen = 1,10-phenanthroline, has been studied. The pa-dependent changes in the charge-transfer absorption and emission spectra for both complexes are attributed to protonation of the quinoxalinedithiolate ligand at the imine nitrogen. For Pt(qdt)(2)(2-), Single protonation leads to a large red shift (2582 cm(-1)) of the low-energy charge-transfer-to-dithiolate absorption band and the emergence of a new red-shifted emission feature at 728 nm. In solutions of pH below 6, a second protonation takes place at the other qdt ligand, yielding Pt-(Hqdt)(2). This neutral complex has been isolated and characterized by H-1 NMR spectroscopy and elemental analyses. The ground-state basicity constants of the two protonation steps have been determined from spectrophotometric titrations to be PKb1 = 6.9 and PKb2 = 8.4 Thermodynamic estimation of the excited state PKb1* using a Forster analysis yields values ranging from 1.1 to 1.6, although the apparent pK(b1)*, Obtained by spectrophotometric titration of the emission, is much closer to that of the ground state due to the sub-nanosecond lifetimes of Pt(qdt)(2)(2-) and Pt(Hqdt)(qdt)(-) and the kinetics of excited state protonation. For the neutral Pt(phen)(qdt) complex, protonation leads to small changes in the absorption spectrum and quenching of the solution emission. The different protonation behavior between the two complexes results from differences in their electronic structures.