In this paper the solubility and optical activity of Bi12SiO20 (BSO) single crystals grown in aqueous NaOH under hydrothermal conditions were reported. BSO was found to have a positive solubility coefficient from 350 to 400 degrees C in 0.5-3 M NaOH, as well as a positive dependence on mineralizer concentration. The reason why additional SiO2 should be introduced in the mineralizer to grow high quality BSO was analyzed. High quality and colorless BSO crystals were grown using 4M NaOH as a mineralizer and additional 3 wt% SiO2 introduced in the mineralizer. Results showed that the optical activity of hydrothermal colorless BSO was superior to that of BSO grown by the Czochralski (Cz) method. (C) 2012 Elsevier B.V. All rights reserved.