Glucose isomerase has been found for the first time to catalyze the isomerization of glucose to fructose in the ionic liquid N, N-dibutylethanolammonium octanoate (DBAO). Isomerization was achieved at temperatures of 60-80 degrees C although a substantial amount of mannose was formed at elevated temperatures via the Lobry-de Bruyn-van Ekenstein transformation. Complete recovery of the sugars after reaction was achieved by extraction with aqueous HCl, thus making the protocol attractive for continuous operation.