Addition of 2,4-dimethylbenzenesulfenyl chloride to sialic acid glycal gives crystalline 2-chloro-3-thiosialic acid 3 in 85% yield. Reaction of 3 with sodium thiomethoxide in acetonitrile at 0 degrees C affords the sialic acid donor alpha-2-(methylthio)-3-thiosialic acid 4 in quantitative yield. Sialylation of glycosyl accepters 9 and 10 with 4 in the presence of phenylsulfenyl triflate (PST) as promotor in CH3CN at -40 degrees C gives alpha-sialosides in good yield and excellent stereoselectivity. No beta-sialosides are formed in either case. Removal of the auxiliary 3-(2,4-dimethylphenyl)-thio group is achieved in high yield using Ph(3)SnH and AIBN in refluxing toluene. Protected GM(3) trisaccharide and 6-sial-2-yllactose were obtained on a gram scale.