The oxidation of KI to Is by sucrose culture media of Aspergillus niger is due to H2O2 formed by Muller''s glucose oxidase system. The reaction is inhibited by catalase but not by KCN, etc., and occurs in the presence of the xanthine oxidase and other primary oxidation systems. CH3CHO is similarly oxidized to CH3COOH. These secondary oxidations are quantitative.