The treatment of α- and β- D-xylopyranosyl chloride 2,3,4-tri(chlorosulfate) and the corresponding α-D-lyxo derivative with a catalytic quantity of sodium iodide in methanol solution resulted in the removal of the chlorosulfate groups and rapid methanolysis of the anomeric chlorine atoms. The methanolysis proceeded with predominant inversion in each case (80–90 %), independent of the orientation of the C-2 substituent. In the presence of sodium methyl mercaptide, however, thioglycosides having an inverted configuration were produced from 1,2-cis chlorides, and thioglycosides having an unchanged anomeric configuration were formed from 1,2-trans chlorides. It is proposed that all these glycosidations occur via a free pentopyranosyl chloride intermediate, and that under the basic conditions of thioglycosidation a 1,2-anhydro intermediate is formed from 1,2-trans chlorides.