Ultramafic volcanism is one model which fits currently available data for some eruptions on Jupiter's moon, Io. Assuming that such activity does occur, it is possible to apply komatiitic analogs and terrestrial ultramafic flow models to Io. We have used such analogs and models along with Galileo data to investigate the nature of ultramafic materials that are assumed to occur at high-temperature hotspots on Io. The unusual komatiites of the Commondale greenstone belt, South Africa, are consistent with available Galileo data on the temperatures and composition of potential Ionian ultramafic materials. These komatiites had high SiO2 (~50 wt %) and MgO (~31 wt %) contents, which are inferred to result in high liquidus temperatures (~1610 ¿C), low dynamic viscosities (~0.2 Pa s), and low densities (~2680 kg/m3) and in the crystallization of orthopyroxene phenocrysts (not found in other komatiites). We used the Commondale composition and the model of Williams et al. <1998> to investigate potential ultramafic flow behavior on Io. Compared to their terrestrial counterparts, Ionian ultramafic lava flows would have less potential for turbulent flow, lower maximum emplacement distances, and lower thermal erosion rates because of the lower Ionian gravity and their more silicic composition. Shallow (10 m) thermal erosion channels seem less likely on Io, unless (1) lavas are superheated, (2) lavas are more ultramafic (>31% MgO), (3) eruption rates, eruption durations, or flow volumes are high, or (4) substrates are unconsolidated, partly consolidated, or volatile-rich and thus more erodable. If volatiles are present in the lava as vesicles, then increasing lava vesicularity results in decreasing maximum emplacement distances, decreasing thermal erosion rates, and decreasing erosion channel depths for given eruption durations. ¿ 2000 American Geophysical Union