This is the first report on the Ferrier rearrangement of 2-halo-glycals with S-nucleophiles. We present herein the selective synthesis of new 2-iodo-2,3-unsaturated O- and S-glycosides. We obtained in good yields and high anomeric selectivity a-glycosydes. As a particular behaviour, we also describe heteroaromatic thio sugar derivatives were the C3 addition was performed in an alternative mechanism. A complete structure and conformation analysis by NMR was also presented.