A method of restricting the production of C 4 oxygenates in an anaerobic fermentation of a gas substrate comprising at least one of CO and a mixture of CO 2 with hydrogen, the method comprising: a. passing the gas stream to an anaerobic fermentation zone containing at least one species of anaerobic microorganism capable of producing an liquid product comprising C 2 or C 3 oxygenates; wherein said microorganism is selected from at least one species of homoacetogenic or heteroacetogenic microorganism; b. converting at least a portion of the gas stream to the liquid product by contact of the microorganism in the fermentation zone with the gas stream; c. providing at least one crotonate-like compound in the fermentation liquid in an amount effective to restrict production of C 4 oxygenates, said crontonate-like compound represented by the structural formula I. R 2 (R 3)C=C C R 1 or II. R 2 -A-C R 1 wherein: R 1 is -OH, -OR 5, or -N(R 4) 2, wherein R 5 is hydrocarbyl and each R 4 may be the same or different and is hydrogen or hydrocarbyl; R 2 is hydrogen, -NH 2, -OH or -CX 3 wherein each X may be the same or different and is hydrogen or halogen; R 3 is hydrocarbyl of 1 to 18 carbons which may be unsubstituted or substituted with -OH, alkoxyl of 1 to about 6 carbons, or halogen; A is an aromatic moiety having 5 or 6 ring atoms which may be all carbon atoms or may be heterocyclic with one hetero atom selected from the group consisting of oxygen and nitrogen which aromatic moiety has -R 2 at the 2 position wherein -R 2 is as defined above and which may be unsubstituted or substituted at one or more of the higher carbon positions with (i) hydrocarbyl of 1 to 18 carbons which may be substituted with -OH, alkoxyl of 1 to 6 carbons, or halogen, (ii) -OR 6 wherein R 6 is lower alkyl, (iii)-N(R 7) 2 wherein each R 7 is the same or different and is hydrogen or hydrocarbyl or (iv) halogen; with the proviso that in structural formula I, at least one of R 2 and R 3 is or is substituted with haloalkyl and that in formula II at least the 2 or 3 carbon position of the aryl moiety is substituted with an electron withdrawing group selected from the group consisting of halogen and haloalkyl; d. withdrawing fermentation liquid containing the liquid product from the fermentation zone; and e. recovering the liquid product from the fermentation liquid. a. passing the gas stream to an anaerobic fermentation zone containing at least one species of anaerobic microorganism capable of producing an liquid product comprising C 2 or C 3 oxygenates; wherein said microorganism is selected from at least one species of homoacetogenic or heteroacetogenic microorganism; b. converting at least a portion of the gas stream to the liquid product by contact of the microorganism in the fermentation zone with the gas stream; c. providing at least one crotonate-like compound in the fermentation liquid in an amount effective to restrict production of C 4 oxygenates, said crontonate-like compound represented by the structural formula I. R 2 (R 3)C=C C R 1 or II. R 2 -A-C R 1 wherein: R 1 is -OH, -OR 5, or -N(R 4) 2, wherein R 5 is hydrocarbyl and each R 4 may be the same or different and is hydrogen or hydrocarbyl; R 2 is hydrogen, -NH 2, -OH or -CX 3 wherein each X may be the same or different and is hydrogen or halogen; R 3 is hydrocarbyl of 1 to 18 carbons which may be unsubstituted or substituted with -OH, alkoxyl of 1 to about 6 carbons, or halogen; A is an aromatic moiety having 5 or 6 ring atoms which may be all carbon atoms or may be heterocyclic with one hetero atom selected from the group consisting of oxygen and nitrogen which aromatic moiety has -R 2 at the 2 position wherein -R 2 is as defined above and which may be unsubstituted or substituted at one or more of the higher carbon positions with (i) hydrocarbyl of 1 to 18 carbons which may be substituted with -OH, alkoxyl of 1 to 6 carbons, or halogen, (ii) -OR 6 wherein R 6 is lower alkyl, (iii)-N(R 7) 2 wherein each R 7 is the same or different and is hydrogen or hydrocarbyl or (iv) halogen; with the proviso that in structural formula I, at least one of R 2 and R 3 is or is substituted with haloalkyl and that in formula II at least the 2 or 3 carbon position of the aryl moiety is substituted with an electron withdrawing group selected from the group consisting of halogen and haloalkyl; R 1 is -OH, -OR 5, or -N(R 4) 2, wherein R 5 is hydrocarbyl and each R 4 may be the same or different and is hydrogen or hydrocarbyl; R 2 is hydrogen, -NH 2, -OH or -CX 3 wherein each X may be the same or different and is hydrogen or halogen; R 3 is hydrocarbyl of 1 to 18 carbons which may be unsubstituted or substituted with -OH, alkoxyl of 1 to about 6 carbons, or halogen; A is an aromatic moiety having 5 or 6 ring atoms which may be all carbon atoms or may be heterocyclic with one hetero atom selected from the group consisting of oxygen and nitrogen which aromatic moiety has -R 2 at the 2 position wherein -R 2 is as defined above and which may be unsubstituted or substituted at one or more of the higher carbon positions with (i) hydrocarbyl of 1 to 18 carbons which may be substituted with -OH, alkoxyl of 1 to 6 carbons, or halogen, (ii) -OR 6 wherein R 6 is lower alkyl, (iii)-N(R 7) 2 wherein each R 7 is the same or different and is hydrogen or hydrocarbyl or (iv) halogen; d. withdrawing fermentation liquid containing the liquid product from the fermentation zone; and e. recovering the liquid product from the fermentation liquid.