The calcium metallacrown Ca(II)[15-MCCu(II)N(Trpha)-5](2+) was obtained by self-assembly of Call, Cu-II, and tryptophanhydroxamic acid. Its X-ray structure shows that the core calcium ion is well-encapsulated in the five oxygen cavity of the metallacrown scaffold. The kinetics of Ca-Ln core metal substitution was studied by visible spectrophotometry by addition of Ln(III) nitrate to solutions of Ca(II)[15-MCCu(II)N(Trpha)-5](2+) in methanol solution at pH 6.2 (Ln(III) = La-III, Nd-III, Gd-III, Dy-III, Er-III) to obtain the corresponding Ln(III)[15 -MCCu(II)N(Trpha)-5](3+) complexes on the hours time scale. The reaction is first order in the two reactants (second order overall) with different rate constants across the lanthanide series. In particular, the rate for the Ca-Ln substitution decreases from La-III to Gd-III and then increases slightly from Gd-III to Er-III. This substitution reaction occurs with second order rate constants ranging from 0.1543(3) M-1 min(-1) for La-III to 0.0720(6) M-1 min(-1) for Gd-III. By means of the thermodynamic log K constants for the same reaction previously reported, the rate constants for the inverse Ln-Ca substitution were also determined. In this study, we demonstrated that the substitution reaction proceeds through a direct metal substitution and does not involve the disassembly of the MC scaffold. These observations in concert allow the proposition of a hypothesis that the dimension of the core metals play the major role in determining the rate constants of the substitution reaction. In particular, the largest lanthanides, which do not require complete encapsulation in the MC cavity, displace the Ca-II ion faster, whereas in the back reaction Ca-II displaces the smaller lanthanides faster as they interact relatively weakly with the metallacrown oxygen cavity.