ABSTRACT: The potential mode of inhibition for xalic acid on polyphenol oxidase (PPO) was investigated. The extent of inhibition was influenced not only by oxalic acid concentration but also by pH. Inhibition was most prominent at pH 4.0 where complete inhibition occurred at the 4-mM oxalic acid concentration and was less evident at higher pH values. Inhibition of PPO by oxalic acid was due to its binding with copper to form an inactive complex, and the inhibition was characterized as noncompetitive. Oxalic acid diminished the catechol-quinone product formation, and no quinone bleaching was observed. Oxalic acid was a more potent inhibitor of PPO compared with other structurally related acids.