Interaction between Cs+ and meso-octamethylcalix[4]pyrrole (1) was investigated. Electrospray ionization mass spectrometry was used. Cation Cs+ forms with ligand 1 the complex 1·Cs+. By employing DFT calculations, four conformations of 1·Cs+ were derived. Ligand 1 can be considered as a macrocyclic receptor for Cs+.