The kinetics of the hydroformylation of 1-octene in a supercritical carbon dioxide medium, catalyzed by a tris(3,5-bis[trifluoromethyl]phenyl)phosphine-modified rhodium catalyst, have been investigated. The influence of the concentration of carbon dioxide, reactants, catalyst precursors, and the reaction temperature has been determined. A kinetic model was developed, which describes the concentration-time profiles of the reactants, the linear and branched aldehydes, and the internal alkenes. Using the kinetic model activation energies for hydroformylation of 1-octene to nonanal and 2-methyloctanal were determined. Throughout the concentration ranges studied an approximate first order dependence of the hydroformylation rate on the hydrogen and catalyst concentration was found which indicated that oxidative addition of hydrogen was the rate limiting step. The increase in reaction rate and regioselectivity with an increase in ligand concentration is a striking feature of the catalyst investigated here. At higher concentrations the reaction rate was found to have a strong negative order dependence on the carbon monoxide concentration. The reaction rate had a positive order in 1-octene at a concentration lower than 0.5 mol L(-1) while saturation kinetics were observed at a higher concentration. The results were explained by invoking the contribution of both monophosphine and diphosphine rhodium species to the hydroformylation catalysis. (C) 2011 Elsevier B.V. All rights reserved.