Anion binding by the pyroborates (catB) 2 O (1, cat=O 2 C 6 H 4 -1,2) and (S,S-Ph 2 C 2 H 2 O 2 B) 2 O (2) has been investigated by spectroscopic, structural and titration methods. 1 has been shown to act as a bifunctional Lewis acid, exemplified by the complementary (1:1) binding of bidentate bases such as acetate and dihydrogen phosphate. The former complex has been characterized in the solid state by X-ray diffraction and a binding constant of 1500±550M −1 determined in chloroform solution. The reaction of 2 with acetate, by contrast, leads to breakdown of the Lewis acid chelate and to the formation of the homochiral borate anion [(S,S-Ph 2 C 2 H 2 O 2 ) 2 B] − in good yield (84% based on the chiral component).