Photophysical properties such as absorption and emission spectra, lifetimes, and redox potentials of eight ruthenium complexes, Ru(LL)(2)(MebpyCOOH)(2+), where LL represents bpy, phen, Me(2)bpy, Me(4)bpy, (MeO)(2)bpy, (EtO)(2)bpy, Cl(2)bpy, and NO(2)phen, have been measured. The acid dissociation constants of ground and excited states have been determined. The ground-state pK(a) values were obtained from the pH dependence of the complex absorbance changes. The excited-state pK(a)* values were extracted from the emission titration curve and corrected for the excited-state lifetime of both protonated and deprotonated species. The largest Delta pK(a), pK(a)* - pK(a), found for Ru(Me(2)bpy)(2)(MebpyCOOH)(2+) and Ru(Me(4)bpy)(2)(MebpyCOOH)(2+) of 1.7 indicate that MebpyCOOH gains most of the MLCT excited-state electron. The big negative Delta pK(a) found for Ru(Cl(2)bpy)(2)(MebpyCOOH)(2+), -4.2, clearly indicates the metal-to-ligand charge transfer to the Cl(2)bpy ligands.