Thioglycosides have been employed extensively for the synthesis of complex oligosaccharides, carbohydrate libraries, and mimetics of O-glycosides. While very useful, aglycon transfer is a problematic side reaction with thioglycosides. In this paper, a series of mechanistic studies are described. The aglycon transfer process is shown to affect both armed and disarmed thioglycosides, cause anomerization of the carbon-sulfur bond of a thioglycoside, and destroy the product of a glycosylation reaction. The results indicate that the aglycon transfer process can be a major problem for a wide range of thioglycosides. This side reaction is especially important to consider when carrying out complex reactions such as solid-phase glycosylations, one-pot or orthogonal multicomponent glycosylations, and construction of carbohydrate libraries. To prevent transfer, a number of modified aglycons were examined. The 2,6-dimethylphenyl ( DMP) aglycon was found to effectively block transfer in a variety of model studies and glycosylation reactions. The DMP group can be installed in one step from a commercially available thiol ( 2,6-dimethylthiophenol) and is useable as a glycosyl donor. On the basis of these features, the DMP group is proposed as a convenient and improved aglycon for thioglycosides.