Two uranyl squarates, (UO2)(6)(O4O4)(3)(OH)(6)O-2 center dot 9H(2)O.4NH(4) (1; a = 16.6897(7) angstrom, cubic, /23) and (UO2)(C4O4)-(OH)(2)center dot 2NH(4) (2; a = 8.5151(4), b = 15.6822(8), c = 7.3974, orthorhombic, Pbcm), have been synthesized from ambient aqueous solutions as;3 function of pH. Oligomerization of the uranyl cation from monomeric pentagonal bipyramids (pH < 5) to [(UO2)(3)O(OH)(3)] trimers (5 < pH < 8) in 1 and ultimately [(UO2)(OH)(2)](n) chains (7 < pH < 8) in 2 is observed. This evolution of speciation versus pH is consistent with what has been observed in solution and thus may be represented by the uranyl hydrolysis equilibrium, mUO(2)(2+) + nH(2)O <-> [(UO2)(m)(OH)(n)](2mn-n) + nH(+). Structural systematics, physical properties, and a discussion of species selectivity by squarate anions are presented.