A spectrophotometric cell for measurements of solubility and slow reaction rate in supercritical carbon dioxide (scCO(2)) up to 30 MPa was developed. A free piston made of a Teflon-coated magnet in the cell is moved up and down in the cell cylinder by an outside magnet, which is automatically driven by a gas cylinder at an arbitrary time interval. There is a one-way path in the free piston, and solution flow in the opposite direction is prevented; thus, the solution is stirred by circulating scCO(2) in the looped pass. Using the developed cell, the solubilities of 5, 10, 15, 20-tetrakis(pentafluorophenyl)porphyrin (H(2)tpfpp) in scCO(2) were determined at various temperatures and pressures. The dynamic process of copper(II) ion incorporation into the porphyrin core for the reaction of H(2)tpfpp with Cu(hfac)(2) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate) was monitored by simultaneously dissolving both reagents in scCO(2) in order to evaluate the applicability of the developed spectrophotometric cell for rate measurements of slow chemical reactions in scCO(2). (C) 2001 Elsevier Science B.V. All rights reserved.