Studies of the new tetra-amido macrocyclic ligand (TAML) activator [Fe-III{(Me2CNCOCMe2NCO)(2)CMe2}OH2](-) (4) in water in the pH range of 2-13 suggest its pseudo-octahedral geometry with two nonequivalent axial H2O ligands and revealed (i) the anticipated basic drift of the first pK(a) of water to 11.38 due to four electron-donating methyl groups alongside (ii) its counterintuitive enhanced resistance to acid induced iron(III) ejection from the macro cycle. The catalytic activity of 4 in the oxidation of Orange II (S) by H2O2, in the pH range of 7-12 is significantly lower than that of previously reported TAML activators, though it follows the common rate law (v/[Fe-III] = k(I)k(II)[H2O2] [S]/(k(I)[H2O2] + k(II)[S]) and typical pH profiles for k(I) and k(II). At pH 7 and 25 C the rate constants k(I) and k(II) equal 0.63 +/- 0.02 and 1.19 +/- 0.03 M-1 s(-1), respectively. With these new values for pK(a), k(I) and k(II) establishing new high and low limits, respectively, the rate constants k(I) and k(II) were correlated with plc values of all relations log k = log k(0) + alpha X pK(a) were established with log k(0) = 13 +/- 2 and 20 +/- 4 and alpha = -1.1 +/- 0.2 and -1.8 +/- 0.4 for k(I) and k(II), respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their pK(a) values.