A membrane reactor is presented for homogeneous catalysis in supercritical carbon dioxide with in situ catalyst separation. This concept offers the advantages of benign high-density gases, i.e., the possibility of achieving a high concentration of gaseous reactants in the same phase as the substrates and catalyst as well as easy catalyst localization by means of a membrane. For the separation of the homogeneous catalyst from the products an inorganic microporous membrane is used. The concept is demonstrated for the hydrogenation of 1-butene using a fluorous derivative of Wilkinson's catalyst [RhCl{P-(C6H4-p-SiMe2CH2CH2C8F17)(3)}(3)]. The size of Wilkinson's catalyst, 2-4 nm, is clearly larger than the pore diameter, 0.5-0.8 nm, of the silica membrane. The membrane will, therefore, retain the catalyst, while the substrates and products diffuse through the membrane. Stable operation and continuous production of n-butane has been achieved at a temperature of 353 K and a pressure of 20 MPa. A turnover number of 1.2 x 10(5) has been obtained during 32 h of reaction. The retention of the catalyst was checked using Uv-vis spectroscopy and ICP-AAS; no rhodium or phosphorous species were detected at the permeate side of the membrane. (C) 2003 Elsevier Inc. All rights reserved.