Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru-II(btfmb)(2)(LL)](2+), [Ru-II(dtb)(2)(LL)](2+), and [Ru-II(bpy)(2)(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)(2)-bpy, and dtb is 4,4'-((CH3)(3)C)(2)-bpy, and LL is either dcb = 4,4'-(CO2H)(2)-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long tau > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pK(a)'s of 2.31 +/- 0.07, and their characterization enabled accurate determination of the two pK(a) values for the commonly utilized dcb ligand, pK(a1) = 2.1 +/- 0.1 and pK(a2) = 3.0 +/- 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru-III(btfmb(-))L-2](2+)* localized excited state and a [Ru-III(dcb(-))L-2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pK(a)* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).