The structures of two peracetylated β-D-galactopyranosides substituted at the anomeric C atom with either an isothiocyanate or an azide group, have been determined by single crystal X-ray diffraction analysis. The two compounds show very similar molecular conformation, except for the substituent groups at the anomeric C atom and for the acetyl groups of the O6 atoms. They crystallize in different infinite chain-type structures. While the isothiocyanate group extends in a large intermolecular space, the azide group is placed in a more crowded environment.