By using electrospray ionization mass spectrometry (ESI-MS), it was proven experimentally that the univalent cesium cation (cs(+)) forms with meso-octamethylcalix[4]pyrrole (abbrev. 1) the cationic complex species 1.Cs+. Further, applying quantum chemical DFT calculations, four different conformations of the resulting complex 1.Cs+ were derived. It means that under the present experimental conditions, this ligand I can be considered as a macrocyclic receptor for the cesium cation. (C) 2016 Elsevier B.V. All rights reserved.