Abstract: A process for purifying glucose isomerase comprises the steps of acid treatment and salt fractionation. An enzyme solution is treated with an acid, such as acetic acid, to a pH from about 3.5 to about 5.0. The proteinaceous solids are collected and extracted with a buffer, such as imidazole, whose solution has a pH of about 6 to about 8. The solution is then collected and a salt, such as ammonium sulfate, is dissolved therein from about 40% to about 50% of its saturation point. The proteinaceous solids which form are removed and additional ammonium sulfate is dissolved to attain from about 41% to about 60% of its saturation point, followed by collection of the solids containing purified enzyme. A composition which preserves enzyme activity upon storage of glucose isomerase and which imparts resistance to thermal deactivation of said enzyme comprises an aqueous solution of glycerol, a buffer whose solution is at a pH of about 6 to about 8, divalent cobalt ions and magnesium ions.