Exchange between the SO42- ions in ettringite and borate is believed to occur during borate removal from wastewater by the Ca(OH)(2)-Al-2(SO4)(3) addition method. To investigate borate uptake by ettringite, we synthesized ettringite in the presence of H3BO3 and prepared suspensions of ettringite in H3BO3 solution. At the theoretical Ca(OH)(2) / Al-2(SO4)(3) ratio for ettringite, with an increase in the H3BO3 / Al-2(SO4)(3) ratio, the ettringite concentration decreased but the concentration of amorphous compounds increased; further, the amount of borate uptake by the precipitate formed also increased. Upon alkalization of the H3BO3 -Al-2(SO4)(3) solution to pH = 10 with NaOH solution, borate-containing amorphous Al(OH)(3) was precipitated. It was proposed that the borate coprecipitated along with amorphous Al(OH)(3) (which showed boron uptake) and did not undergo exchange with SO42- ions in ettringite; this was strongly supported by the results of IR and TG analyses performed on the boron-containing precipitates. At a high H3BO3 / Al-2(SO4)(3) ratio, ettringite decomposed to afford amorphous Al(OH)(3), and borate coprecipitation was promoted. Upon suspension in H3BO3 solution, the ettringite precipitate showed low boron uptake, which was presumably due to the substitution of SO42- in ettringite with B(OH)(4)(-).