Recombinant acetogenic Clostridia are engineered to modulate production of aliphatic C2-C6 alcohols and aliphatic C2-C6 organic acids from synthetic gases. One aspect of the invention provides a method of producing an aliphatic C2-C6 alcohol using an acetogenic Clostridium micro-organism having at least one genetic modification that reduces or eliminates C2-C6 carboxylic acid production by the modified organism. Another aspect of the invention provides a method of producing an aliphatic C2-C6 alcohol using an acetogenic Clostridium micro-organism having one or more genetic modifications that cause increased enzyme activity of carbon monoxide dehydrogenase, aldehyde ferredoxin oxidoreductase, NADPH-dependent alcohol dehydrogenase, or alcohol dehydrogenase. Yet another aspect of the invention provides a method of producing aliphatic C2-C6 alcohols using acetogenic Clostridium micro-organisms that have been genetically modified to increase C2-C6 aliphatic alcohol formation and decrease the production of the corresponding C2-C6 aliphatic organic acid.