The invention relates to improvements in the production of alcohols by microbial fermentation, particularly to production of alcohols by microbial fermentation of a substrate comprising CO. It more particularly relates to the addition of an inorganic sulfur source to a fermentation system such that one or more micro-organisms convert a substrate comprising CO to alcohols. In a particular embodiment, a microbial culture is provided with sodium polysulfide, wherein a substrate comprising CO is converted to products including ethanol and 2,3-butanediol.