The partial pressures of Si(g), B(g), BSi(g), BSi2(g), and BSi3(g) were measured over the condensed phase mixtures silicon + silver and silicon + germanium + silicon nitride + silver, contained in boron nitride Knudsen cells. The thermal functions were calculated for the B-Si gaseous species, and the atomization enthalpies Delta(atom)H degrees(m) (in kJ . mol(-1)) were deduced as 312 +/- 12 (BSi), 767 +/- 18 (BSi2), and 1199 +/- 28 (BSi3) at T = 0 K; and 317 +/- 12 (BSi), 777 +/- 18 (BSi2), and 1214 +/- 28 (BSi3) at T = 298.15 K. These values in combination with enthalpies of formation of Si(g) and B(g) yielded enthalpies of formation Delta(f)H degrees(m) (at T = 298.15 K; in kJ . mol(-1)) : 698 +/- 14 (BSi), 688 +/- 20 (BSi2), and 701 +/- 31 (BSi3).