Mercury(II) enhances the incorporation rate of zinc(II) into 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H-2(tpps)(4-)). The detailed kinetics and mechanism for the metal-substitution reaction of Hg-2(tpps)(2-) with zinc(II) was studied at 25 degrees C, I = 0.1 mol dm(-3) (NaNO3), pH 4.5-6.5, [H-2(tpps)(4-)] = (1-4) x 10(-6), [Hg2+] = 10(-5)-10(-3), and [Zn2+] = 10(-4)-10(-2) mol dm(-3). The reaction showed a biphasic kinetic behavior of the following steps : Hg-2(tpps)(2-) + Zn2+ --> Hg(tppsZn2- + Hg2+ --> Zn(tpps)(4-) + 2Hg(2+). The rate equations for the first-step and second-step reactions are expressed as -d[Hg-2(tpps)(2-)]/dt = k(1)k(2) [Zn2+][OH-][Hg-2(tpps)(2-)](k(-1)[HgOH+] + k(2)[Zn2+])(-1) and -d[Hg(tppsZn2-]/dt = k(3) + k(4)[H+], respectively. The values of the rate constants were found to be k(1) = (1.02 +/- 0.04) x 10(8) mol(-1) dm(3) s(-1), k(2)/k(-1) = (0.14 +/- 0.02), k(3) = (1.09 +/- 0.02) x 10(-2) s(-1), and k(4) = (3.00 +/- 0.09) x 10(3) mol(-1) dm(3) s(-1) at 25 degrees C, and the activation enthalpies and entropies were Delta H-double dagger = 102 +/- 6 kJ mol(-1), Delta S-double dagger = 251 +/- 12 J mol(-1) K-1 for k(1); Delta H-double dagger = 0 kJ mol(-1), Delta 5(double dagger) = -266 +/- 16 J mol(-1) K-1 for k(2)/k(-1); Delta H-double dagger = 81 +/- 2 kJ mol(-1), Delta S-double dagger = -11.9 +/- 0.5 J mol(-1) K-1 for k(3); Delta H-double dagger = 100 +/- 3 kJ mol(-1), Delta S-double dagger = 157 +/- 5 J mol(-1) K-1 for k(4), respectively. The formation of heterodinuclear metalloporphyrin Hg(tppsZn2- was suggested by the following kinetic results. The observed rate constant for the first-step reaction decreased with increase in mercury(II) concentration, increased with the increase in concentrations of zinc(II), and leveled off at high zinc(II) concentration. The observed rate constant for the second-step reactions was characteristic of a dissociation of Hg(tppsZn2- : the rate constant was independent of zinc(II) and mercury(II) concentrations and of hydrogen ion concentration at high pH. A series of absorption spectra for the first- and second-step reactions supported the binding of mercury(II) and zinc(II) in the intermediate.