The synthesis of formyl-methanofuran and the reduction of the heterodisulfide (CoM-S-S-CoB) of coenzyme M (HS-CoM) and coenzyme B (HS-CoB) are two crucial, H2 -dependent reactions in the energy metabolism of meth-anogenic archaea. The bioenergetics of the reactionsin vivo were studied in chemostat cultures and in cell suspensions of Methanothermobacter thermautotrophicusmetabolizing at defined dissolved hydrogen partial pressures (pH2 ).