Volcanic plumes deposit magmatic pyroclasts and SO2 frost on the surface of Io. We model the plume activity detected by Galileo at the Pillan and Pele sites from 1996 to 1997 assuming that magmatic eruptions incorporate liquid SO2 from near-surface aquifers intersecting the conduit system and that the SO2 eventually forms a solid condensate on the ground. The temperature and pressure at which deposition of solid SO2 commences in the Ionian environment and the radial distance from the volcanic vent at which this process appears to occur on the surface are used together with observed vertical heights of plumes to constrain eruption conditions. The temperature, pressure, and density of the gas--magma mixtures are related to distance from the vent using continuity and conservation of energy. Similar eruption mass fluxes of order 5 ¿ 107 kg s-1 are found for both the Pillan and the Pele plumes. The Pele plume requires a larger amount of incorporated SO2 (29--34 mass %) than the Pillan plume (up to ~6 mass %). Implied vent diameters range from ~90 m at Pillan to ~500 m at Pele. The radial extents of the optically dense, isothermal, incandescent parts of the eruption plumes immediately above the vents are ~100 m at Pillan and ~1300 m at Pele. Gas pressures in the vents are ~20 kPa at Pillan and ~2 kPa at Pele and the eruption conditions appear to be supersonic in both cases, though only just so at Pele.