The photobehavior of complexes of the type Pt(diimine)(mes)(2) is investigated (where diimine = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), and-4,7-diphenyl-1,10-phenanthroline (dpp)and mes the mesityl (2,4,6-trimethylphenyl) anion). For all compounds studied, solution RT emission is observed to be weak and excited-state lifetimes;are found to be short (120 ns) regardless of solvent choice. Evidence is presented for energy-transfer quenching of Pt(dpp)(mes)(2) luminescence in toluene by dissolved O-2 (primarily producing singlet oxygen) with an observed quenching rate constant of k(q) greater than or equal to 1.3 x 10(9) M-1 s(-1). Electron-transfer quenching is also observed in the presence of 3,5-dinitrobenzonitrile, yielding a quenching rate constant of k(q) greater than or equal to 1.6 x 10(9) M-1 s(-1). The latter observation suggests that these Pt(II) systems may have future value as excited-state reductants. All of the complexes display a much more intense and longer-lived luminescence in the solid state at room temperature. Several possible explanations for this dependence on phase are proposed, with the most probable mechanism involving radiationless deactivation in solution via rotation of the o-methyl groups of the mesityl ligands.