The kinetics of the mutarotation of xylose, and series of 6-substituted 6-deoxy-glucoses, and a series of 2-substituted 2-deoxy-glucoses catalysed by oxonium ion, water, 4-substituted pyridines, 2,6-Iutidine, Tris, 2,2′-iminodiethanol, and morpholine have been measured. Electron-withdrawing substituents at the 6-position decreased the rate of mutarotation catalysed by oxonium ion and by water but increased that of the mutarotation catalysed by bases. Electron-withdrawing substituents at the 2-position decreased the rate of mutarotation with all the catalysts studied except in the case of the 2,6-Iutidine-catalysed reaction of 2-amino-2-deoxy-D-glucose hydrochloride, which occurred slightly faster than that of 2-deoxy-D-glucose. The mechanisms of these mutarotations are discussed in the light of these substituent effects, the solvent-isotope effect, and the diffusion-controlled limit of the rate of bimolecular reactions. Steric-hindrance factors were calculated for the 2,6-Iutidine-catalysed reactions but there appears to be no correlation between these and the size of the 2- or the 6-substituents. Intramolecular catalysis was found for the mutarotation of 6-deoxy-D-gluco-hepturonic acid and 6-O-(o-hydroxyphenyl)-D-glucose.