The invention relates to improvements in the production of butanol and butyrate by microbial fermentation, particularly to production of alcohols by microbial fermentation of a substrate comprising CO and the addition of an inorganic sulfur additive. It more particularly relates to the provision of an inorganic organic sulfur source to a fermentation system such that one or more micro-organisms convert a substrate comprising CO to butanol. In one aspect the invention uses a sulfur additive comprising inorganic sulfur compounds having a +2 to a +4 sulfur oxidation state that produces sulfur oxoanions and hydrosulfur oxoanions in an aqueous fermentation medium.