To obtain gluocuronylarabinalic acid which is useful as a metallic ion-screening agent for detergent or a dispersant because of its excellent metallic ion-screening properties and dispersibility by selective oxidation of glucosylarabonic acid prepared by alkaline oxidative carbon-diminishing reaction of starch hydrolyzate.

Starch hydrolyzate is subjected to alkaline oxidative carbon- diminishing reaction to prepare gluocosylarabonic acid. This gluocosylarabonic acid is allowed to react with a hypohalite salt in the presence of a catalytic amount of a secondary or tertiary alicyclic nitroxyl cation to selectively oxidize the whole or a part of the primary alcoholic functional groups in the gluocosylarabonic acid whereby the objective glucuronylarabinarlic acid or its composition having an average degree of polymerization of 2-50, preferably 2-10, containing 100-50%, preferably 99-60% of the anhydroglucuronyl units, ≤1%, preferably ≤0.7%, more preferably 0.5% of free reducing saccharides is obtained.