Oligomannose glycans are of interest as HIV vaccine components, but they are subject to mannosidase degradation in vivo. Herein, we report the synthesis of oligosaccharides containing a thio linkage at the nonreducing end. A thio-linked dimannose donor participates in highly stereoselective glycosylations to afford trimannose and tetramannose fragments. Saturation transfer difference nuclear magnetic resonance (STD NMR) studies show that these glycans are recognized by HIV antibody 2G12, and we confirm that the reducing terminal S-linkage confers complete stability against x. manihotis mannosidase.