Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-), comprising a Co(4)O(4) core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)(3)](3+) (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O(2) production >= 5 s(-1) at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O(2)-H(2)O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.