The dissolution behavior of boron in a molten slag was evaluated at 1773 K to establish the boron removal mechanism from silicon refining for preparing higher purity solar-grade silicon. From the experimental results, a mechanism of boron removal from silicon into the CaO-SiO2-Al2O3 slag system was confirmed to proceed via BO33- and B-6(2-) following a linear relationship with oxygen partial pressure in the slag system. In addition, the distribution ratio of boron was seen to increase with increasing slag basicity. 6B + 1/3O(2-) = B-6(2-) + 1/6O(2)(g) The boride capacity of the CaO-SiO2-Al2O3 slag can be expressed as a linear function of the activity of CaO and the optical basicity. It is suggested that the proposed dissolution mechanism of boron could be appropriate to slag systems, depending on oxygen partial pressure and basicity. (C) 2013 Elsevier Ltd. All rights reserved.