The products obtained by the periodate oxidation of methyl α- and β-D-xylopyranosides have been examined by nuclear magnetic resonance spectroscopy. In deuterium oxide each product exists mainly in two forms, one of which is a hydrated dialdehyde and the other a hemialdal (1,4-dioxane derivative). In dimethyl sulfoxide the dialdehyde cyclizes slowly to yield the hemialdal which, at equilibrium, is found to be by far the most stable of the many isomeric forms possible. The spectrum of this hemialdal in both solvents provides an example of a degenerate ABX type of pattern. The spectra of the products prepared from 5-deuterated xylosides show that the 1-methoxyl group of each hemialdal is axial and the 4-hydroxyl group equatorial, and that the one derived from the β-anomer possesses an inverted chair conformation relative to that of the other hemialdal and of the glycoside itself. The 2-hydroxyl group of each hemialdal appears to be axially oriented. The data support the long-standing view that glycol scission of anomeric aldopentopyranosides leads to products that are mirror images.It has been found also that each product yields a mixture of several p-nitrobenzoates.