The position of the equilibrium between aldoheptoses and their glycosidic anhydrides depends crucially on the configuration of the heptose. Depending on that configuration, the 1,6-anhydropyranose, the 1,7-anhydropyranose, or the 1,6-anhydrofuranose is the major product, its proportion varying from 99% to less than 1%. The position of the equilibrium is predictable from conformational considerations. 1,7-Anhydrofuranoses have not been encountered. The 1,3-dioxane ring of the 1,7-anhydropyranoses was found to assume a skew form.