A new model is proposed to account for the observed properties of Io. The crust is probably siliceous, but the outermost visible surface is composed primarily of several allotropes of elemental sulfur, plus frosts of SO2, H2S and S2O. The 4&mgr;m feature in Io's IR reflectance is identified as the (&ngr;1+&ngr;3) band of SO2 frost. A possible source of the S is dissociation of iron sulfide brought up from the core by mantle convection. The SO2 is produced in the volcanoes and the H2S by interaction of magnetospheric protons with the surface S. Delayed luminescence from S2O could cause the posteclipse brightening. The orbital torus is sustained by evaporation from molten ejecta from the volcanoes. Several additional diagnostic absorption bands and two additional minor torus species are predicted by this model.