In aqueous solution, boric acid  can act as a weak Brønsted acid, that is, a proton donor, with pK ~ 9. However, it more often acts as a Lewis acid, accepting an electron pair from a hydroxyl ion produced by the water autoprotolysis:
:  + 2    +  (pK = 8.98)
This reaction is very fast, with characteristic time less than 10 μs. Polymeric boron oxoanions are formed in aqueous solution of boric acid at pH 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the tetraborate ion , found in the mineral borax:
Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions:
: 2 +    + 3 (fast, pK = —1.92)
: 4 +    + 6 (slow, pK = —2.05)
In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula  where 3x+q = 2y + z will eventually equilibrate in solution to a mixture of , , , and .
These ions, similarly to the complexed borates mentioned above, are more acidic than boric acid itself. As a result of this, the pH of a concentrated polyborate solution will increase more than expected when diluted with water.