The equilibria of self-assembly of 15-metallacrown-5 (15-MC-5) complexes of Cu2+ and (S)-alpha-alaninehydroxamic acid (alpha-Alaha, HL) with the lanthanide (Ln) ions Eu3+ or Gd3+ in aqueous solution are described. The binary Ln(3+)/alpha-Alaha systems were first studied by potentiometric and calorimetric in-cell titrations; the latter technique allowed us to define the most suitable speciation model. On the contrary, because the kinetics of formation of the Ln(3+) 15-MC-5 complexes is slow, their stability constants were determined by out-of-cell (batch) potentiometric titrations. Two 15-MC-5 complexes are formed with both Eu3+ and Gd3+, namely, {Ln[Cu5L5H-5])(3+) and {Ln[Cu5L5H-5](OH)}(2+). with the latter being the hydroxo species of the former. The acidity of the former to give the hydroxo species is remarkably high (log K = 4.40-4.69). Moreover, our potentiometric and spectrophotometric investigations clearly indicate that the hydroxide ion is coordinated to the central Ln ion, as was reported for several 15-MC-5 in the solid state. The formation of {Ln[Cu5L5H-5]}(3+) starts at ca. pH 3.5, which converts at ca. pH 4.5 into the {Ln[Cu-5-L5H-5](OH)}(2+) species, which predominates up to pH 7, where a purple precipitate occurs. The coexistence of both 15-MC-5 species and the copper(II) 12-MC-4 species of alpha-Alaha ([Cu5L4H-4](2+)) was observed under appropriate experimental conditions (pH and ligand and metal concentrations). A complete ESI-MS investigation of the Ln(3+)/Cu2+/alpha-Alaha system at different pH's confirmed the formation of the two 15-MC-5 species. The 15-MC-5 stability constants were employed to quantitatively evaluate the solution behavior of Ln(III) MCs regarding their integrity, ligand substitution, and transmetalation processes, In particular, EDTA or DOTA, added in equimolar amounts, should not appreciably interfere with the MC integrity, as found in previous experimental investigations, although it is expected that at higher amounts of EDTA, the MC should be disrupted. Our results also demonstrate that an excess of alpha-aminohydroxamate does not interfere with the integrity of the MC, and the disappearance of the CD spectra upon addition of the R enantiomer to 15-MC-5 containing the S enantiomer is due to a very rapid ligand exchange with formation of all possible isomers with no selectivity. The stability of the 15-MC-5 complexes in the presence of transferrin, serum albumin, or an excess of Zn2+ is also discussed. With regards to the latter metal ion, we found that the MCs are stable toward Gd3+/Zn2+ transmetalation. although the presence of a phosphate buffer promotes the disruption of the MC scaffold by formation of stable Gd3+/phosphate species.