Boron isotopic fractionation between vapor and brine phases separated under supercritical conditions has been experimentally examined using a Na-Ca-K-Cl fluid. Experiments were conducted at 425, 440, and 450°C. Phase separation was controlled by adjusting pressure. Fractionation between the coexisting phases was less than 0.5%.. In contrast, fractionation between trigonal and tetrahedral B, at these temperatures, has been predicted to be approximately 8%.. The lack of fractionation suggests that the trigonal/tetrahedral speciation of B is similar in the two phases. Although this result may not be general for other fluid compositions, it is relevant to ridge crest hydrothermal solutions and indicates that the boron isotopic compositions of these solutions reflect the proportions of B from seawater and crustal sources and not phase separation.