Escherichia coli succinate-ubiquinone oxidoreductase (SQR) and menaquinol-fumarate reductase (QFR) are excellent model systems to understand the function of eukaryotic Complex II. They have structural and catalytic properties similar to their eukaryotic counterpart. An exception is that potent inhibitors of mammalian Complex II, such as thenoyltrifluoroacetone and carboxanilides, only weakly inhibit their bacterial counterparts. This lack of good inhibitors of quinone reactions and the higher level of side reactions in the prokaryotic enzymes has hampered the elucidation of the mechanism of quinone oxidation/reduction in E. coli Complex II. In this communication DT-diaphorase and an appropriate quinone are used to measure quinol-fumarate reductase activity and E. coli bo-oxidase and quinones are used to determine succinate-quinone reductase activity. Simple Michaelis kinetics are observed for both enzymes with ubiquinones and menaquinones in the succinate oxidase (forward) and fumarate reductase (reverse) reactions. The comparison of E. coli SQR and QFR demonstrates that 2-n-heptyl 4-hydroxyquinoline-N-oxide (HQNO) is a potent inhibitor of QFR in both assays; however, SQR is not sensitive to HQNO. A series of 2-alkyl-4,6-dinitrophenols and pentachlorophenol were found to be potent competitive inhibitors of both SQR and QFR. In addition, the isolated E. coli SQR complex demonstrates a mixed-type inhibition with carboxanilides, whereas the QFR complex is resistant to this inhibitor. The kinetic properties of SQR and QFR suggest that either ubiquinone or menaquinone operates at a single exchangeable site working in forward or reverse reactions. The pH activity profiles for E. coli QFR and SQR are similar showing maximal activity between pH 7.4 and 7.8, suggesting the importance of similar catalytic groups in quinol deprotonation and oxidation.