The effects of temperature, concentration and solvent on the tauto-merit equilibria of glucose and fructose were determined by gas chro-matography (GLC) as TMSi ethers. Four tautomeric forms of fructose as their TMSi ethers were identified by GLC-MS and IR spectroscopy. The results of the GLC measurements indicate that the proportion of the α-anomer at the equilibrium of glucose solutions increased slightly both with increasing concentration and temperature. In pyridine there was somewhat more α-D-glucose at equilibrium than in water solutions. The relative amounts of the fructose tautomers in water solutions varied only slightly as a function of concentration, whereas the amount of the furanose forms increased clearly with increasing temperature. In the pyridine solutions of fructose less pyranose forms were found than in the water solutions at corresponding temperatures, whereas in pyridine solutions at constant temperatures the pyranose forms increased with increasing fructose concentrations.