The OMEGA imaging spectrometer aboard Mars Express has been used to study the evolution of the CO abundance over the Hellas basin, as a function of the seasonal cycle. In spite of significant uncertainties, due to the low intensity of the (2−0) CO band at 2.3 µm, these data allow to search for spatial variations over Hellas, where the surface pressure and the CO band depth are maximum. The OMEGA data suggest an enhancement over Hellas at Ls = 130−150 deg (end of southern winter), by a factor of

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... about 2 with regard to Ls = 330−350 deg (end of northern summer). This behavior is to be compared with the evolution of argon, another non-condensible species which also shows strong variations with Ls at southern latitudes (Sprague et al. 2004) . This result appears also consistent with the GCM predictions which indicate an enrichment by a factor of about 2 of non-condensible species over Hellas during southern winter (Forget et al. 2006) .