A series of [Ru-II(terpy-R)(phen-X)Cl]PF6 complexes was designed where terpy-R is the tridentate 4'-(4-methylmercaptophenyl)-2,2':6'2"-terpyridine ligand MeMPTP and phen-X is a substituted phenanthroline with hydro (I), 5-nitro (2), 5,6-dimethyl (3), and 3,4,7,8-tetramethyl (4). This series allows us to compare the reactivity of phenanthroline-containing procatalysts with that of its well-established bipyridine counterparts as well as to study the effects of electron-withdrawing and -donating substituents on water oxidation. These species were thoroughly characterized by spectroscopic and spectrometric methods, and the structures of 1, 3, and 4 were determined by single-crystal X-ray diffraction. The procatalysts 1-4 show opposite trends compared to known terpyridine/bipyridine species; the unsubstituted procatalyst 1 yields a turnover number (TON) of 410 followed by 250 and 150 for complexes 3 and 4 with electron-donating substituents. Species 2, with electron-withdrawing properties, yields the lowest TON of 60. Although the TONs decrease upon substitution, the presence of electron-donating methyl substituents enhances the rate of O-2 evolution during an early stage of catalysis. Interestingly, no evidence of conversion from chlorido-containing procatalysts into expected aqua-containing catalysts was observed for 1-4 by NMR and UV-visible spectroscopy during the induction period. This observation, along with reactivity toward (NH4)(2)[Ce-IV(NO3)(6)], suggests that water nucleophilic attack happens to a high-valent ruthenium species rather than while at the Re oxidation state. Reactivity follows a trend similar to the rate of O-2 evolution in all complexes. Furthermore, the electrospray ionization mass spectrometry and H-1 NMR analyses of 1, as recovered after catalysis, indicate the presence of a chlorido ligand.