Encapsulation of guests in molecular containers has implications for chemical transformations, sensing, stimuli-responsive devices, separations and design of molecular materials. In organic media, various strategies have been developed to build molecular containers that assemble through non-covalent interactions. By contrast, the design of water-soluble containers is still a great challenge and methods to gain an active control on the encapsulation and/or transport of molecules are topics of interest [1]. We have recently shown that a tetra cationic calix[4]arene receptor (TAC4, 1a) recognizes organic anions and forms supramolecular capsules in water at physiological pH. Appropriate gemini guests, having two aromatic and negatively charged head groups (BSCn), trigger the selfassembling of homodimeric capsules in the presence of TAC4 [2]. Capsule formation in water has been confirmed by 1H and DOSY NMR, MS and ITC. 1a 1b Schematic of TAC4 and BSCn (1a) and the Gd3+ metallacrown (1b) The formation of capsular entities is not restricted to calixarenes. Metallacrowns (MC) may selectively recognize anions through their open metal sites (1b) [3]. Here we also report on the formation of dimeric compartments of Gd3+[15-MCCu(II), S-pheHA-5] metallacrown with organic carboxylates in neutral aqueous solution. In both cases, the analysis of the host-guest adducts with HypCal, a general-purpose computer program for the determination of standard reaction enthalpy and binding constant values by calorimetry [4], provides key information and addresses the issues related to compartment size and selectivity.