1. The over-all course of the aerobic reduction of cytochrome c by xanthine oxidase was investigated under various conditions, especially to clarify the mechanism of reoxidation of reduced cytochrome c. The higher the concentration of enzyme or hypoxanthine and the lower the pH, the more marked was the reoxidation of reduced cytochrome c. 2. The reoxidation was also dependent upon the properties of the cytochrome c sample. The more marked reoxidation was observed with samples having greater reactity with CO. 3. With Amberlite CG-50 column chromatography, Keilin-Hartree's cytochrome c preparation gave two enzymically reducible and less reoxidizable monomeric fractions, and a monomeric and a polymeric fraction. The latter two fractions were less active as electron acceptors and are thought to be mainly responsible for the reoxidation. 4. Reoxidation of reduced cytochrome c has been found to be caused in the following two ways : the peroxidation due to modified cytochrome c originally contaminating and also probably formed during the reaction, and the acceleration of peroxidation by uric acid which is the oxidation product of the substrate.