A collectorgenerator (CG) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O-2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)(2) (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2 ''-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O-2 formation by the catalyst; the O-2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O-2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O-2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The CG technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)(3)(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3 , HPO42, imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3 and HPO42 facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O-2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive CH oxidation of the bases. O-2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO43 and at pH 7 in a 0.1 M H2PO4 /HPO42 buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the RuIV(O)(2+) form of the catalyst gave O-2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by RuV(O)(3+), gave O-2 with an efficiency of 100%. The lower efficiency for RuIV(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.