Gaseous SO2 was discovered on Io, the innermost galilean satellite of Jupiter, by the IRIS IR Michelson interferometer1 on board Voyager 1 in March 1979. This SO2 is likely to be associated with eruptions of the seven active volcanoes discovered at the same time by the Voyager cameras2. The UV spectrum of the solar light reflected by the surface of Io should be modified by the transmission S̄(λ) of a SO2 atmosphere and by the reflectivity r (λ) of the surface. Both gaseous SO2 and solid3 SO2 present a deep absorption band around 290 nm. We have examined a published spectrum of Io taken in 1978 by the IUE Earth-orbiting telescope4, and found an absorption dip centred around 290 nm in the albedo curve. We attribute this dip to gaseous SO2, with a possible contribution of solid SO2. However, the absorption dip is smaller than if the gaseous SO2 quantity measured by Voyager 1 in the IR were uniform. It can be better represented by an SO2 frost controlled gaseous atmosphere and a temperature distribution of Io's surface compatible with observations.