Boric acid may be prepared by reacting borax (sodium tetraborate decahydrate) with a mineral acid, such as hydrochloric acid:
: ·10 + 2 HCl → 4  + 2 NaCl + 5 
It is also formed as a by product of hydrolysis of boron trihalides and diborane:
:  + 3  →  + 3 HX (X = Cl, Br, I)