NOVELTY - Anaerobic bioconversion of a gas substrate comprising carbon monoxide, hydrogen and carbon dioxide to an oxygenated organic compound in a fermentation broth, involves continuously introducing the gas substrate into a fermentation broth containing microorganisms, maintaining pH of the fermentation broth, supplying metal cation capable of forming water-insoluble complex with cyanide anion, withdrawing substrate-depleted gas phase from the fermentation broth and continuously or intermittently withdrawing oxygenated organic compound from the fermentation broth.    USE - The method is useful for biologically converting a gas substrate comprising carbon monoxide, hydrogen and carbon dioxide to an oxygenated organic compound in a fermentation broth under anaerobic conditions, where the oxygenated organic compound comprises alkanol chosen from at least one of ethanol, propanol and butanol (all claimed) for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels.    ADVANTAGE - The method protects the microorganisms from hydrogen cyanide contained in the syngas that passes to the fermentation broth. The rate of formation of the insoluble complex is sufficiently high. The amount of cyanide that is taken up by microorganisms does not result in an undue adverse effect on the population of microorganisms. The method is environmentally-friendly and cost-effective.    DETAILED DESCRIPTION - Anaerobic bioconversion of a gas substrate comprising carbon monoxide, hydrogen and carbon dioxide to an oxygenated organic compound in a fermentation broth, involves continuously introducing the gas substrate into a fermentation broth containing microorganisms suitable for bioconverting the substrate to an oxygenated organic compound, the fermentation broth is maintained under anaerobic fermentation conditions, and the gas substrate have sufficient contact with the fermentation broth to provide an oxygenated organic compound-containing fermentation broth and a substrate-depleted gas phase, maintaining the pH of the fermentation broth at 4-6, continuously or intermittently supplying to the fermentation broth a metal cation of one or more of iron, cobalt, nickel and zinc capable of forming a substantially water insoluble complex with a cyanide anion, to provide the metal cation dissolved in the fermentation broth in an amount at least stoichiometrically sufficient to form under the anaerobic fermentation conditions and pH, the water insoluble complex with the cyanide anion if introduced with the gas substrate into the fermentation broth, continuously withdrawing the substrate-depleted gas phase from the fermentation broth having a substantial absence of hydrogen cyanide, and continuously or intermittently withdrawing a portion of the fermentation broth for recovery of the oxygenated organic compound, where the withdrawal is sufficient to maintain the oxygenated organic compound in the fermentation broth below a concentration that unduly adversely affects the microorganisms, where the gas substrate contains from time to time a hydrogen cyanide.