Abstract: A process for producing L-ascorbic acid or D-erythorbic acid, or in each case its sodium, potassium or calcium salt, from 2-keto-L-gulonic acid or 2-keto-D-gluconic acid, or in each case its sodium, potassium or calcium salt, that involves incubating 2-keto-L-gulonic acid or 2-keto-D-gluconic acid, each as the free acid or as its sodium, potassium or calcium salt, and the cells of a thermoacidophilic microorganism at temperatures from about 30° C. to about 100° C., and at a pH from about 1 to about 6, in a solution, to form L-ascorbic acid or D-erythorbic acid or an appropriate salt thereof, and isolating said product from the solution.