Ubiquinones (UQs) and menaquinones (MKs) perform distinct functions in Escherichia coli. Whereas, in general, UQs are primarily involved in aerobic respiration, the MKs serve as electron carriers in anaerobic respiration. Both UQs and MKs can accept electrons from various dehydrogenases, and may donate electrons to different oxidases. Hence, they play a role in maintaining metabolic flexibility in E. coli whenever this organism has to adapt to conditions with changing redox characteristics, such as oxygen availability. Here, the authors report on the changes in both the size and the redox state of the quinone pool when the environment changes from being well aerated to one with low oxygen availability. It is shown that such transitions are accompanied by a rapid increase in the demethylmenaquinone pool, and a slow increase in the MK pool. Moreover, in exponentially growing cultures in a well-shaken Erlenmeyer flask, it is observed that the assumption of a pseudo-steady state does not hold with respect to the redox state of the quinone pool.