A process for producing a conjugated di- or poly- unsaturated fatty acid having from 12 to 24 carbon atoms, or a salt or ester thereof, comprises reacting a non-conjugated free fatty acid, or a salt or ester thereof, with a base in the presence of a solvent comprising a monohydric alcohol having from 1 to 6 carbon atoms, wherein the reaction is carried out at a temperature of from 120.degree. C. to 200.degree. C. in the presence of water in an amount of at least 4% by weight based on alcohol.