Carboxydotrophic acetogenic microorganisms do not produce MEK and/or 2-butanol. They lack the biosynthesis pathways to make these products. In addition, they produce the intermediate (R,R)-2,3-butanediol whereas the production of MEK and 2-butanol requires production of the intermediate (R,S)-2,3-butanediol. Nonetheless, the production of MEK and/or 2-butanol can be accomplished using recombinant microorganisms adapted to express or overexpress key enzymes in the MEK and/or 2-butanol biosynthesis pathways. Such microorganisms, such as the carboxydotrophic acetogen Clostridium autoethanogenum, can ferment substrates comprising CO. The overall scheme involves the production of 2-butanol from (R,S)-2,3-butanediol and the conversion of (R)-acetoin to (S)-2,3-butanediol. These steps are involved in the production of both MEK and 2-butanol. Such fermentation methods offer a means of using carbon monoxide from industrial processes which would otherwise be released into the atmosphere and pollute the environment.