A high-pressure stopped-flow instrument with spectrophotometric detection has been developed, which enables us to follow fast reactions with half-lives longer than 10 ms in supercritical carbon dioxide (scCO(2)) at various temperatures up to 150 degreesC and pressures up to 30 MPa. The rate of the metalloporphyrin formation reaction in scCO(2) between 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin and bis(1,1,1,5,5,5-hexaflucoro-2,4-pentanedionato)copper(II) (Cu(hfac)(2)) was successfully measured under a variety of excess concentrations (Ccu) of Cu(hfac)(2)Based on the dependence of the conditional first-order rate constants on C-Cu, the metalation mechanism was evaluated in scCO(2). (C) 2001 Elsevier Science B.V. All rights reserved.