Computed spectra of the Martian atmosphere transmission as observed by the ISO SWS show the possibility of detecting the dust level altitude during a global dust storm. Dust composition can also be derived from the analysis of the spectral response. A Target of Opportunity proposal is accepted in order to realise these scientific objectives. Unfortunately, the ISO solar elongation requirement limits extremely the Mars observation possibilities and the chances of observing before the end of ISO lifetime are now very low. The historical Martian storm season begins only at the middle of October 1997 and the atmosphere observed until the end of September 1997 shows an exceptional clarity.