In view of the very few mechanistic information available on the complex formation between lanthanide ions and calixarenes, we study here the reaction mechanism leading to the formation of the bimetallic complex between EuIII and p-tert-butylcalixarene (b-LH8). We show that in dmf, the 1 : 1 complex [Eu(b-LH6)(NO3)(dmf)4] partially dissociates into a compound A which is an important intermediate in the formation of bimetallic [Eu2(b-LH2)(dmf)5]. Several spectroscopic (ES-MS, visible, luminescence, FT-IR) and conductometric techniques are combined with variable-temperature and variable-pressure experiments to demonstrate that in absence of base, intermediate A is the cationic species [Eu(b-LH6)(dmf)4]+. The thermodynamic parameters of the dissociation reaction, K5(298) = (4 ± 1) × 10−5 M, ΔH0 = +16 ± 3 kJ mol−1, ΔS0 = −30 ± 11 J mol−1 K−1, as well as the reaction volume ΔV0 = +9.4 ± 0.9 cm3 mol−1 are evaluated and rationalized. In presence of triethylamine, intermediate A deprotonates to generate the neutral species A′ = [Eu(b-LH5)(dmf)4], which, in turn, further reacts with europium nitrate to yield the bimetallic complex. In addition, the pKas of the parent calixarene LH8, of b-LH8 and of p-iso-propylcalixarene are determined in dmf, as well as the association constant between b-LH8 and triethylamine, Kassoc = (9 ± 1) × 103 M−1.