Sulphur isotope fractionation factor is a critical parameter describing evolution of sulphur concentration and isotopes in a natural magmatic system. However, currently available values are tied to analogue systems and models. An eXort has been made to establish a model for equilibrium sulphur isotope fractionation in a magma with sulphides, by conducting laboratory experiments. To this end, a new and safe gas-mixing furnace using the gas mixture CO-CO -SO 2 has been designed to simulate desired fO -fS 2 conditions at temperatures up to 1600 °C [1]. Experiments were carried out using ~Fo 90 crystals as capsules with basalt powder and 99.99% Fe powder as the starting materials. A dominant proportion of sulphur in this system is acquired from the gas. Experiments were carried out under fO2 conditions ranging from -8.5 to -11.1 (ΔQFM -0.65 to -3.27) and fS2 conditions ranging from -1.1 to -1.8 at 1200 – 1400 °C. The samples were analysed for their major element compositions using an EPMA and δ 34S using a SIMS. Time-series experiments demonstrate that the system reaches equilibrium, both in terms of major element and sulphur isotope composition within 8 hours. From over 25 equilibrium experiments, each lasting 8 hours, the fractionation observed is significant and follows a trend, higher the fO 2 and lower the temperature, the larger the fractionation is. At 1300 °C, the fractionation (Δ 34 S ) is as high as 4.36 ± 0.47 ‰ for a melt-sulphide fO of -9 (ΔQFM -1.79) and tends to 0 at a fO 2 of -10.71 (ΔQFM -3.49). This fractionation is significantly greater than what is predicted by analogue models [2]. Our experimentally determined equilibrium sulphur fractionation model could explain the evolution of δ 34 S from -1 to +5.6 ‰ observed among the lower crustal cumulates of the Talkeetna arc [3], in which sulphide precipitation under oxidising conditions raises the δ 34 S of the magma. Thus, δ 34S trends in cumulates may be an indicator of magmatic redox condition.