The borate ion, B(OH)4-, is capable of forming anionic complexes of both 1:1 and 1:2 stoichiometries with bidentate chelating ligands. The reactions are B(OH)4- + H2X <-> BX- + 2H2O (K1) and BX- + H2X <-> BX2- + 2H2O (K2). Reactions with 1,2-ethanediol, 1,2-propanediol, glycolic acid, lactic acid, and oxalic acid are reported. The successive reactions of the diols with B(OH)4- are shown to be thermodynamically similar. In marked contrast, the successive addition of the more acidic alpha-hydroxy carboxylic acids is characterized by very different thermodynamic parameters for each step. Compared with the first reaction, addition of a second ligand has a much less favorable enthalpy of reaction which is only partially balanced by a much more favorable entropy of reaction. The net result accounts for the experimental observation that the ratio K1/K2 is very large for these ligands. The dramatic difference in the thermodynamics of second ligand addition for acidic ligands reaches its limit with oxalic acid, in which case addition of a second ligand does not occur. Thermodynamic results are also presented for several mixed-ligand systems. Thermodynamic parameters were determined from the temperature dependence of K1 and K2 as measured by B-11 NMR spectroscopy.