Metallocapped complexation of the thallium(I) ion with calix[4]crown-6 (1) and three different calix[4]biscrown-6 derivatives (2-4) has been investigated by a combination of Tl-203 and H-1 NMR in a CD3CN/CDCl3 (4:1 v/v) solution. The results clearly revealed the formation in solution of mono- and dithallium(I) complexes in which the metal cations are held in the ligands' cavities close to the calix[4]arene ring. In addition, a solid state mononuclear complex between 1,3-calix[4]bis-o-benzo-crown-6 (3) and TlClO4 was prepared, and its X-ray crystal structure was determined. The results of the structural data in solution and the solid state suggested that the calix[4]crown-6 derivatives considered provide pi cavities as active sites for the complexation of thallium(I) ions.