The kinetics and mechanism of the metal(II) ion incorporation into a porphyrin by the reaction of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (H₂tpfpp) with the β-diketonate complexes of nickel(II), copper(II), and zinc(II) ions in supercritical carbon dioxide have been spectrophotometrically studied. The saturation dependence was observed for the conditional-first-order rate constants as a function of the mole fraction of the excess β-diketonate complexes. Such dependence indicates that the metalation reaction consists of a fast preequilibrium for the formation of the outer-sphere association complex prior to the rate-determining step for the metal ion incorporation into the porphyrin core. The mechanistic consideration and specific cosolvent effects are described on the basis of the variation in rates in the presence of cosolvents such as water and alcohols.