Ruthenium terpyridine complexes of the type [Ru^II(trpy)(L)(X)][ClO₄]_n1–15 [trpy = 2,2′∶6,2′′-terpyridine; L = NC₅H₄NNC₆H₄(R), R = H, m-Me, m-Cl, p-Me or p-Cl; X = Cl⁻, n = 1 (1–5); H₂O, n = 2 (6–10) or OH⁻, n = 1 (11–15)] have been synthesized. The single crystal structures of 1 and 6 were determined. Complex 4 develops in two possible isomeric forms whereas all other complexes stabilise preferentially in one isomeric form. The complexes exhibit strong MLCT bands near 500 nm and ligand based transitions in the UV region. 1–5 exhibit moderately strong emissions at 77 K near 600 nm. The chloro (1–5) and the hydroxo (11–15) complexes display ruthenium(III)–ruthenium(II) couples and three to four successive one-electron ligand based reductions. The aqua-complexes (6–10) exhibit a reversible 2e⁻/2H⁺ single-step oxidation process in the pH range 1–5.5 corresponding to the [Ru^II(trpy)(L)(H₂O)]²⁺–[Ru^IV(trpy)(L)(O)]²⁺ couple and the potential decreases linearly with increase in pH. The chemical oxidations of 6–10 by an excess of Ce^IV in 0.5 M H₂SO₄ also lead to the formation of corresponding [Ru^IV(trpy)(L)(O)]²⁺. The oxo-complexes are stable only in the presence of an excess of Ce^IV, otherwise they catalyse the oxidation of water to dioxygen and convert back into the parent aqua-species. The pseudo first order rate constant of the process [Ru^IV(trpy)(L)(O)]²⁺ → [Ru^II(trpy)(L)(H₂O)]²⁺ has been determined.