We report the first detection of the forbidden 10.51 &mgr;m line of S3+ in the Io plasma torus (IPT). The observations were performed with European Space Agency's Infrared Space Observatory. Seven measurements were acquired, four at the dusk ansa and three at the dawn ansa. Variations with &lgr;III have been investigated, and an upper limit for variation of 20% at the dusk ansa was determined. Using this value, a maximum possible change of 8--10% from the nominal value of electron temperature with &lgr;III has been calculated. Alternatively, the density of hot electrons can vary by 20%. The absolute intensity of S(IV) was found to be 1.5--3.5 times higher than that predicted by a model of the IPT for a Voyager-like torus. Possible reasons for this difference have been investigated. It was found that an increased electron temperature, a higher concentration of suprathermal electrons, or a higher plasma density can lead to an intensity increase of a factor of 1.5. To increase the intensity by a factor of 3.5 it is necessary to reduce the electron temperature and at the same time increase the plasma density. Additionally, we report a detection of an S(III) line at 33.48 &mgr;m. ¿ 2001 American Geophysical Union