In succinoxidase, CoQ1 had low activity, and CoQ2 through CoQ10 had comparable activities in both the acetone- and pentane-extracted systems. In the DPNH-oxidase system, CoQ1 had low activity; CoQ2, CoQ3, and CoQ4 had about one-fourth the activity; and CoQ5 and CoQ6 had about one-half the activity of CoQ10. CoQ7, CoQ8, and CoQ9 had activity comparable with that of CoQ10. Hexahydro- and octahydrocoenzyme Q4 had activities comparable with that of CoQ4 and CoQ10 for succinoxidase, but about 10–15% that of CoQ10 for DPNH-oxidase. The double bond in the isoprenoid unit adjacent to the nucleus, and possible cyclization, are not necessary for activity. 2,3-Dimethoxy-5-methyl-6-heptadecyl-l,4-benzoquinone shows succinoxidase and DPNH-oxidase activities comparable with that of hexahydrocoeuzyme Q4. The 5-methyl group is not essential for succinoxidase activity, but has some significance for DPNH-oxidase. A tertiary hydroxyl group in the isoprenoid unit adjacent to the nucleus abolishes activity for succinoxidase but not completely for DPNH-oxidase. Rhodoquinone had no activity in the succinoxidase system and 25% that of CoQ10 in the DPNH-oxidase system. Representatives of biosynthetic precursors, 2-methoxy-5-methyl-6-phytyl-1,4-benzoquinone and 2-methoxy-3-hydroxy-5-methyl-6-phytyl-1,4-benzoquinone, had no activity in either the succinoxidase or DPNH-oxidase systems. Four compounds having a mechanistic relationship between CoQ and vitamin E had no activity. These data support the concept of two sites for the electron transfer of CoQ. The site for CoQ in succinoxidase is not very specific in the structural and steric requirement for the isoprenoid side chain, and has an electron potentiality that is not met by rhodoquinone. The site for CoQ in DPNH-oxidase has a rather specific requirement for a given length of side chain, and an electron potentiality that is significantly met by rhodoquinone.