Quantum-chemical calculations show how low barriers to anomerization and shifting equilibria cause a significant presence of different monosaccharide isomers in high-temperature processes such as pyrolysis. The transition between isomeric forms of monosaccharides is long-studied, but examination has typically been limited to the solution phase and to pyranose isomers. Processes and rates of anomerization by reversible, gas-phase ring-opening and-closing reactions were predicted for the monosaccharides D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, and D-glucuronic acid. Structures and thermochemistry were computed for stable species and pericyclic transition states using CBS-QB3, and high-pressure-limit Arrhenius reaction parameters were predicted and fitted from 300 to 1000 K. Activation energies for the ring-opening reactions were 162-217 kJ/mol for four-center pericyclic separation of the lactol group but were reduced by catalytic participation of a hydroxyl group within the monosaccharide or an external R-OH group represented by an explicit water molecule, reaching activation energies as low as 97 and 67 kJ/mol, respectively. Equilibrium constants implied increasing fractions of furanose and linear aldehyde anomers with increasing temperature.