Selective double bond isomerization of linoleic acid to cis-9,trans-11-conjugated linoleic acid and trans-10,cis-12-conjugated linoleic acid (CLA) was performed batchwise under kinetic control at 165 degreesC in an n-decane solution. Commercial H-2-preactivated Ru/Al2O3 catalyst (6.19 wt.% Ru) and two H-2-preactivated Ru/Al2O3 catalyst samples that were prepared by incipient wetness impregnation of gamma-Al2O3 with two different precursors of ruthenium, i.e. ruthenium(III) chloride trihydrate (RuCl3.3H(2)O, 3.27 wt.% Ru) and ruthenium(III) acetylacetonate (Ru(acac)(3), 2.97 wt.% Ru), were investigated to study the effect on catalytic activity and isomerization selectivity of residues remaining on the catalyst surface from the impregnation step. Catalyst characterization was done by nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and temperature programmed desorption of hydrogen techniques. Impregnation of gamma-Al2O3 with ruthenium(III) acetylacetonate resulted in a remarkably higher selectivity towards the desired cis-9,trans-11- and trans-10,cis-12-conjugated linoleic acid isomers compared to that shown by impregnation of gamma-Al2O3 with ruthenium(III) chloride trihydrate. (C) 2004 Elsevier B.V. All rights reserved.