The H2 uptake by washed suspensions of chemolithotrophically grown Knallgasbacteria is three to four times larger with carbon dioxide than without it. Electrontransport proceeds at a maximum rate only if endergonic CO2 fixation is made possible. Two types of inhibition experiments were carried out: a) without CO2, b) with CO2. In the absence of CO2, 2.4-dinitrophenol accelerated the H2 uptake by only 20%; thiazinpigments such as methylene blue or thionine caused typical uncoupling reactions leading to an increased H2 uptake by a factor of two to three. In the presence of CO2, sulfhydryl group poisons such as monoiodoacetate or monoiodoacetamide, which inhibit the intermediary processes of CO2 fixation, decrease the rate of H2 uptake to that of the CO2 free control. If the CO2 fixation, inhibited by monoiodoacetate, is separated from the hydrogen transport chain by the addition of methylene blue then the rate of H2 uptake increases again.