A method for the preparation and purification of conjugated linolenic acids is described. The method comprises blending a mixture of vegetable oils and or fats including various concentrations of alpha or gamma and or both linolenic acids with a base. The method transforms approximately over two thirds of .alpha.-linolenic acid (9Z,12Z,15Z-octadecatrienoic acid) into 9Z,11E,15Z-octadecatrienoic acid and 9Z,13E,15Z-octadecatrienoic acid. The method also transforms gamma-linolenic acid (6Z,9Z,12Z-octadecatrienoic acid) into 6Z,8E,15Z-octadeccatrienoic acid and 6Z,10E,12Z-octadecatrienoic acid. In all cases, geometrical isomers and fully conjugated isomers are also produced.