Cellobiose dehydrogenase from Phanerochaete chrysosporium produces H2O2 by electron transfer between cellobiose and O-2 with a lower yield than the 1:1:1 molar ratio displayed by Aspergillus niger. glucose oxidase in the similar reaction between glucose and O-2. The discrepancy could best be explained if both a Fenton’s reaction and the spontaneous reactivity of the oxygen species formed were taken into account.