Sugar ketals are prepared by reacting a sugar with a ketone in the presence of an acid catalyst by a process in whichA. a molecular boron trifluoride compound or trifluoromethanesulfonic acid is used in an amount of only from 0.01 to 10% by weight, based on the sugar used,B. up to a 30-fold molar excess of the ketone is used andC. the water formed during the reaction is removed continuously from the reaction mixture.In a particularly advantageous embodiment of the process, the boron trifluoride is used in the form of boron trifluoride etherate and, when the reaction is complete, this compound or the trifluoromethanesulfonic acid is rendered ineffective under non-aqueous conditions, after which the ketone is evaporated off and the reaction mixture is then subjected to fractional distillation. The process is particularly important for the reaction of L-sorbose with acetone to obtain 2,3:4,6-di-O-isopropylidene-L-sorbofuranose, which is required for the synthesis of ascorbic acid.