Promoted with a small amount of Me4Sn, W(O-2,6-C6H3X2)(2)Cl-4 (X = Ph, Cl) complexes are highly active metathesis catalysts for polyunsaturated fatty esters. Upon metathesis of methyl linoleate, substrate conversions of ca. 95 and 78% were obtained in 2h with the W(O-2,6-C6H3Ph2)(2)Cl-4/Me4Sn and W(O-2,6-C6H3Cl2)(2)Cl-4/Me4Sn catalytic systems respectively. ne lower activity of the W(O-2,6-C6H3Cl2)(2)Cl-4/Me4Sn catalytic system was attributed to the highly deactivating nature of the Cl substituents on the o,o'-position of the aryloxide ligand. Highly electron-withdrawing Cl substituents cause the tungsten atom to be electron-deficient and thus more susceptible to attack by the electron-rich ester groups. However, the W(O-2,6-C6H3Cl2)(2)Cl-4/Me4Sn system was more selective towards dicarboxylic esters. The latter are interesting starting materials for the synthesis of polyesters and polyamides. The W(O-2,6-C6H3X2)(2)Cl-4/Me4Sn catalytic systems were rapidly deactivated by bromine-containing olefinic esters. Deactivation was attributed to a side reaction between the Br-group and the electron-deficient W-centre. (C) 2003 Elsevier B.V. All rights reserved.