A new family of platinum(II) complexes of the form (PtLSR)-S-n have been prepared, where L-n represents a cyclometalating, N<^>C<^>N-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of (PtLSR)-S-1 where HL1 = 1,3-bis(2-pyridyl)benzene, and those of (PtLSR)-S-2, where HL2 = methyl 3,5-bis(2-pyridyl)benzoate. Each group consists of five complexes, where R = CH3, C6H5, p-C6H4-CH3, p-C6H4-OMe, p-C6H4-NO2. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of (PtLCl)-Cl-n with the corresponding potassium thiolate KSR in solution at room temperature. The replacement of the chloride by the thiolate ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in the absorption spectra, not present in the spectra of (PtLCl)-Cl-n. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of (PtLCl)-Cl-n in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to pi(S)/d(pt) -> pi(N<^>C<^>N)* charge-transfer transition from the thiolate/platinum to the N<^>C<^>N ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of (PtLCl)-Cl-n complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the parent complex, according to the region of the spectrum examined.