During the last 15 years, numerous studies have been devoted to the
 determination of the surface composition and temperature of Io, and
 their variation with longitude. In the near infrared (2-5 microns) SO2
 frost is firmly identified, but the identifications of H2S and H2O are
 only tentative. In the mid-infrared S8, SO3, Na2SO3, NaHSO4 and Na2SO4
 are tentatively identified with features seen in the Voyager IRIS
 spectra. Sulfur allotropes, alkali sulfides, and polysulfur oxides have
 also been proposed to explain the UV and visible spectra of Io.
 A clear identification of several of these molecules can be achieved
 with IR observations covering simultaneously a wide spectral range.
 Concerning the SO2 frost coverage, the current studies do not agree on
 the fractional area covered by SO2 and on the amplitude of their
 variations with Io's longitude. Regarding temperature, a better
 knowledge of the spatial distribution of the different types of
 regions of Io (ranging from large cold (100-130 K) regions to small
 hot spots (300-650 K)) are necessary for the study of the Io volcanism
 and of the surface-atmosphere exchanges.
 The goals of this observing program are (1) the positive identification
 of new molecules with simultaneous observation of absorption and
 emission features, (2) the determination of their abundances and their
 longitudinal variations, (3) the study of the surface and subsurface
 temperatures of the large and cold regions covered by SO2, and (4) the
 determination of the temperature and of the heat flow radiated from
 thermal anomalies. These issues can be addressed by ISO by
 (i) performing one high resolution spectroscopic observation in the
 near infrared (2.4-4.55 microns), (ii) measuring the whole mid-infrared
 spectrum (2.4-45 microns) at degraded spectral resolution, and (iii)
 repeating the mid-infrared spectral observation to study variations
 with Io's longitude. The very large spectral coverage of ISO is a
 necessary requirement to achieve all these scientific goals.