Dithioacetals of aldopentoses (D-arabinose, D-ribose, D-xylose, and D-lyxose), aldohexoses (D-glucose, D-mannose, D-galactose), and common oligosaccharides (cellobiose, lactose, gentibiose, melibiose, maltose, and maltotriose) were conveniently prepared by reacting the corresponding free sugars respectively with benzenethiol and/or 1,3-propanedithiol at room temperature in 90% trifluoroacetic acid in much better yields than by the conventional methods.