Heterogeneous supported metal catalysts were applied for the isomerization reaction of linoleic acid (cis-9,cis-12-octadecadienoic acid) to conjugated linoleic acid (CLA) between 80 and 120degreesC in a diluted system. Ru, Ni, Pd, Pt, Rh, Ir, Os, and bimetallic Pt-Rh supported by activated carbon, Al2O3, SiO2Al2O3, MCM-22, H-MCM-41, H-Y, and H-beta were screened. Catalyst characterization was done by XRD, X-ray fluorescence, XPS, H-2-TPD, DCP-AES, and nitrogen adsorption techniques. The isomerization reaction was conducted batchwise in both 1-octanol and n-decane as solvents. Reactions taking place were the isomerization of linoleic acid to CLA, hydrogenation of linoleic acid and CLA to monounsaturated octadecenoic acids (oleic acid, elaidic acid, cis-vaccenic acid, and trans-vaccenic acid), as well as the further hydrogenation of monounsaturated acids to stearic acid (n-octadecanoic acid) with isomerization and hydrogenation being two competing parallel reactions. The isomerization reaction was enhanced by catalyst preactivation under hydrogen. The presence of chemisorbed hydrogen increased the conversion but reduced the isomerization selectivity. Ru and Ni showed the best isomerization properties whereas Pd favored the double bond hydrogenation reaction. (C) 2002 Elsevier Science B.V All rights reserved.