We report here a new catalytic water oxidation system based on Cu(II) ions and a remarkable countercation effect on the catalysis. In a concentrated fluoride solution at neutral to weakly basic pHs, simple Cu(II) salts are highly active and robust in catalyzing water oxidation homogeneously. F- in solution acts as a proton acceptor and an oxidatively robust ligand. F- coordination prevents precipitation of Cu(II) as CuF2/Cu(OH)(2) and lowers potentials for accessing high oxidation-state Cu by delocalizing the oxidative charge over F- ligands. Significantly, the catalytic current is greatly enhanced in a solution of CsF compared to those of KF and NaF. Although countercations are not directly involved in the catalytic redox cycle, UV-vis and F-19 nuclear magnetic resonance measurements reveal that coordination of F- to Cu(II) is dependent on countercations by Coulombic interaction. A less intense interaction between F- and well-solvated C-s(+) as compared with Na+ and K+ leads to a more intense coordination of F- to Cu(II), which accounts for the improved catalytic performance.