Palladium-catalyzed Heck reaction of iodobenzene and styrene was investigated in compressed CO2 using different fluorinated phosphine compounds as ligands at a temperature of 70degreesC. The reaction mixture is a single phase at 12 MPa but biphasic at 8 MPa, a little higher than the critical pressure of pure CO2 under the reaction conditions used. Although the solubility of fluorinated ligands is very high in dense CO2, they have marginal improvements in Heck conversion in this medium compared with a non-fluorinated ligand of triphenylphosphine. The activity of palladium complexes strongly depends on the kind of phosphine compound used, in the order of bis(pentafluorophenyl)phenylphosphine (III) > triphenylphosphine (1), tris(pentafluorophenyl)phosphine (IV) > diphenyl(pentafluorophenyl)phosphine (11), tris(p-fluorophenyl)phosphine (V) > tris(p-trifluoromethyl phenyl)phosphine (VI), 1,2-bis [bis(pentafluorophenyl)phosphino] ethane (VII), for the homogeneous reaction at 12 MPa. This order of effectiveness of these ligands is different from those obtained in conventional organic solvents. Hexane, toluene, ethanol, and N-methylpyrrolidone (NMP) showed maximum conversions with the ligands VI, IV, V and VII, respectively. The conversion in CO2 with the ligand III is comparable with those in polar solvents of ethanol and NMP, and larger than those in hexane and toluene in the presence of the best ligands. The dense CO2 may affect the specific activity of palladium complex catalysts and/or the reactivity of reacting species. Small quantities of fluorinated products were observed to form at high pressure of CO2 and this is direct evidence of P-C bond cleavage during Heck reaction in dense CO2. The activity of palladium complexes with those ligands is higher in more polar solvent. (C) 2002 Elsevier Science B.V. All rights reserved.