The present invention relates to methods for increasing the efficiency of anaerobic fermentation processes (1) that produce acetate as a by-product in addition to a desired product, and (2) that can utilise hydrogen and/or carbon dioxide in the fermentation. The method comprises the steps of converting acetate produced by the fermentation process into hydrogen gas and carbon dioxide gas, and utilising hydrogen gas and/or carbon dioxide gas obtained from the acetate conversion in the anaerobic fermentation process. In particular aspects, the invention relates to processes of producing alcohols, particularly ethanol.