The dissociation constants of the 4-phosphonato-2,2':6',2 "-terpyridine ligand were measured by pH-NMR and potentiometric titration methods. The 4-phosphonato-2,2':6',2 "-terpyridine ligand shows three measurable pK(a) values at 5.99, 4.65, and 3.55 using the latter method. The fourth pK(a) was estimated to be below 2. Spectrophotometric titration of the ruthenium complex [Ru(P-terpy)(Me(2)bpy)(NCS)] (1) (P-terpy = 4-phosphonato-2,2':6',2 "-terpyridine and Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine) exhibits two ground-state pK(a) values at 6.0 and below 4.0, which can be assigned to pK(a1) and pK(a2), respectively. The emission maximum of complex 1 shows a blue shift with decreasing pH, and apparent pH-dependent excited-state lifetimes. Complex 1 shows two excited-state pK(a)* values at 6.5 (pK(a1)*) and below 4.5 (pK(a2)*). Comparison of the ground- and excited-state pK(a) values of complex 1 demonstrates that the excited state has metal to 4-phosphonato-2,2':6',2 "-terpyridine ligand charge-transfer character.