AbstractHitherto, lower growth yields were found with acidophilic methylotrophic bacteria, than those found with neutrophilic methylotrophic ones. Since differences with respect to the oxidative phosphorylation might be responsible for this phenomenon, P/0 quotients were determined for the oxidation byAcetobacter methanolicussp. MB 70 of endogenous substrates, methanol and other substrates. P/0 quotients for endogenous substrates were calculated from the formation of “energy‐rich” phosphate bonds and the concomitant oxygen uptake on transition of the cells from anaerobiosis to aerobiosis in the range between 0.07 to 0.17 nmol P/nmol 0. P/0 quotients for methanol between 0.1 to 0.25 were determined under aerobic conditions by intracellular energization. For this purpose, the energy charge was decreased to values lower than 0.5 by changing the extracellular pH from 4.0 to 7.0, and increased again to values from 0.7 to nearly 1.0, by adding methanol. The addition of FCCP and DCCD2) totally prevented the increase of the ATP concentration. The energization of the same cell preparation produced similar P/0 quotients for methanol, ethanol and glucose. But in a certain cell preparation, the P/0 quotient for glucose (0.01) was considerably lower than that for methanol (0.18); this was the only quotient reduced from 0.12 to 0.05 by aerating the former cell preparation for 4 h. The results are discussed in terms of the coupling of dehydrogenase reactions, for endogenous and exogenous substrates, to the cytochromes of the respiratory chain. Its composition and structure before cytochrome c appears to be responsible for the fact that the growth yields of these bacteria are smaller than expected for such a type of methylot