In the present work we studied the ability of new lasalocid ester with ethylene glycol (Las4) to form complexes with Li þ and Na þ cations using ESI mass spectrometry, 1 H, 13 C NMR and FT-IR spectroscopic methods. The ESI-MS spectra indicate that Las4 forms stable 1:1 complexes with both cations, independent of the stoichiometry in which they were added to the ester. Even different cone voltage values did not influence this stoichiometry. However, increasing cone voltages, lead to degradation of the complexes and formation of various fragmentary cation complexes, still in a 1:1 stoichiometry. In acetonitrile solution, the NMR and FT-IR spectra of the 1:1 complexes of Las4 with Na þ and Li þ , indicate the existence of both, the keto and enol tautomeric forms. The PM5 semiempirical calculations are in agreement with the spectroscopic data. They allow the calculation of the interatomic distances between the cations and the coordinating oxygen atoms as well as visualisation of the structures of the Las4 -Li þ and Las4 -Na þ complexes. The structures indicate the existence of two structural and functional different parts of the ester complex. One part includes the salicylic group, which is stabilised by two intramolecular hydrogen bonds and does not contribute to the complexation process. The second part contains five oxygen atoms, strongly interacting with the cations.