The Galileo ultraviolet spectrometer has observed Jupiter's volcanically active moon to in the 2100--3200 ¿ wavelength range at 13.7 ¿ resolution. We find that both sulfur dioxide frost on the surface and sulfur dioxide gas in lo's atmosphere are detectable. lo's SO2 atmosphere is not hemispherically homogeneous, but is patchy, with both thick and thin regions. During an observation near 120--150 ¿W longitude, we find that a thick SO2 atmosphere (column density N=1.0¿1019 cm-2) covers ~35% of the observed region, while a thinner SO2 (N=4.0¿1017 cm-2) atmosphere covers 25% of the observed region. The thick atmospheric component is significant in the 2350--2400 ¿ range, while the thinner component is important particularly at wavelengths shorter than ~2300 ¿. Likely sources of the SO2 atmosphere are volcanic plumes and sublimation of SO2 frost, as well as outgassing associated with the several hotspots in the field of view. ¿ 1999 American Geophysical Union