We use thermodynamic models and published 1996 Hubble Space Telescope (HST) observations of SO2, SO, and monatomic sulfur gas in the Pele volcanic plume on Jupiter's moon Io to evaluate the temperature (1440 K) and oxidation state (3.3 log fO2 units below the Ni-NiO buffer) of Pele's magma and exsolved volcanic gas. Combination of these results with 1999 HST data on the SO2/S2 ratio in the Pele plume allows us to calculate pressures (10-4.7--10-5.4 bar) in the vicinity of volcanic vent and to present a detailed chemical model for the plume. Our model indicates that the Pele plume represents volcanic gas, which last equilibrated at magmatic temperature and was not significantly altered in the plume. Finally, the redox state of Pele plume indicates that Io is differentiated and has an oxidized, Fe-metal free mantle. ¿ 2000 American Geophysical Union