Selective double bond isomerization of linoleic acid to cis-9,trans-11-conjugated linoleic acid and trans-10,cis-12-conjugated linoleic acid on commercial non-preactivated and H-2-preactivated Ru/Al2O3 catalyst was studied under kinetic control in an agitated batch reactor (glass) at 135-165 degrees C under atmospheric pressure of nitrogen in n-decane solution. Catalyst characterization was done by nitrogen adsorption, X-ray photoelectron spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis, and hydrogen temperature programmed desorption techniques. Over such catalyst, the network involves six reaction: (1) double bond migration of linoleic acid to conjugated linoleic acid, (2) positional and geometric isomerization of conjugated linoleic acid, (3) double bond hydrogenation of linoleic acid to monoenoic acids, (4) double bond hydrogenation of conjugated linoleic acid to monoenoic acids, (5) positional and geometric isomerization of monoenoic acids, and (6) double bond hydrogenation of monoenoic acids to stearic acid. The concentrations of chemisorbed hydrogen and an adsorbed key intermediates on the ruthenium surface influenced the catalytic activity and the selectivities toward isomerization and hydrogenation products through a complex relation. Although presence of hydrogen promoted the side reaction double bond hydrogenation, the isomerization rate was enhanced by chemisorbed hydrogen. (c) 2005 Elsevier B.V All rights reserved.