The influence of boron concentration (CB/mass%) on the surface tension of molten silicon has been investigated with the sessile drop method under oxygen partial pressure P-O2 = 1.62 x 10(-25)-2.63 x 10(-22) MPa, and the results can be summarized as follows. The surface tension increases with CB in the range below 2.09 mass%, and the maximum increase rate of the surface tension is about 30 mN m(-1) (mass% C-B)(-1). The temperature coefficient of the surface tension, (partial derivative(sigma)/partial derivativeT)C-B, was found to increase with the boron concentration in molten silicon. At the interface between molten silicon and the BN substrate, a discontinuous Si3N4 layer was reckoned to form and the layer might prevent BN from dissolving into the molten silicon. Since dissolved boron from the BN substrate into the molten silicon is below 0.054 mass% and the associated increase in surface tension is below 1.5 MN m(-1), the contamination from the BN substrate on the surface tension can be ignored. The relation between the surface tension and C-B indicates negative adsorption of boron and can be well described by combining the Gibbs adsorption isotherm with the Langmuir isotherm. (C) 2003 Elsevier Inc. All rights reserved.