Despite the recent detection of Io's SO2 atmosphere at millimetre wavelengths, several puzzling aspects remain. Modelling of these data suggest a hot, temporally stable, atmosphere with low fractional coverage consistent with upper limits set by UV observations. However, the model is difficult to reconcile with the Voyager 1 IRIS first detection of SO2 made in 1979 at 7.38 microns. IRIS provided only a point measurement and no disc-integrated measurement (which could be compared directly with mm and UV measurements) has yet been made. Millimetre observations suggest a small east-west (trailing- leading) asymmetry in the atmospheric pressure which can be tested by observations at several orbital phases. We propose to use SWS to observe the 7.38 micron line and measure the column abundance and temperature of the atmosphere at 6 orbital phases. Observations of high ionization states in the Io plasma torus provide strong constraints on the equilibrium of the system. The observed presence of OIII, for example, cannot be explained by a chemical equilibrium at the electron temperatures found well inside Io's orbit. Unlike the other major species in the torus, SIV, has no optical emissions. It is also controversial in that IUE observations of the 1406 A intercombination line gives a line intensity 4 times lower than predicted from Voyager 1 UV measurements. This line is relatively weak requiring many hours of observing time and smearing out longitudinal asymmetries (which may be quite strong) in SIV emission. The IR line of SIV at 10.53 microns is 40 times stronger and easily detectable with ISO. We therefore propose to use SWS to observe the longitudinal variability of SIV, compare it to SIII emission (which can be obtained simultaneously), and investigate the chemical equilibrium of the torus.