The cyclic trimer anions dissociate almost completely in aqueous solution giving mainly tetrahydroxyborate anions: 
Other molecules and anions, such as , , , and  are less than 5% at 26 °C.
In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure  with negative charges on the oxygens and a positive charge on the boron, or  with negative charge on the boron. However, this claim has been disproved.