A relationship between the sulphur valence of a melt and f 02 has been determined. The relationship has been used to determine the f O2 conditions under which melt inclusions were trapped in andesitic magmas before magma mixing, and of a slowly cooled pyroclastic flow in which Fe-Ti oxide phases have re-equilibrated. The results help distinguish two trends in lavas from Lascar Volcano: In one, the melt fO2 is buffered by iron redox ratio, while in the other f02 is buffered by S02-H2S in a comagmatic vapour phase. The behaviour of sulphur was experimentally investigated in hydrous phonolitic and rhyolitic melt at 930 °C and 0.5 to 4 kbar. Pyrrhotite is stable under reducing conditions in both melts, and immiscible FeS sulphide liquid is stable under certain conditions of pressure and f S2 at 5 log units above the Ni-NiO buffer. Anhydrite and Srich sodalite are the usual magmatic S-bearing phase under oxidising conditions in rhyolitic and phonolitic melts respectfully. Melt sulphur content is positively correlated with f 02 and f S2. pressure has no significant effect for the conditions investigated. A thermodynamic model has been derived that successfully reproduces the results of this study and of a previous study. The partitioning of sulphur between vapour and melt is a function of f 02, f SZ, phase stabilities and mass balance constraints. Sulphur solubilities and f02 were determined for a suite of back arc basin basalts (BABB). The BABB samples follow two trends: those with low values of f02 have high S contents, whereas more oxidised samples have lower sulphur contents. The solubility behaviour can be described by sulphide-sulphate melt-vapour equilibria. The f02 of the oxidised samples implies that subducted material was incorporated into their magmatic source, a hypothesis supported by major and trace element studies of the samples