The behaviour of naproxen dimer anions ([2Napr-H]-) was studied by electrospray ionization mass spectrometry. Three different analyses were performed, namely direct inlet MS and HPLC/MS (performed for different cone voltages) as well as CID MS/MS (for different collision energies). The results obtained have shown that naproxen dimer anions generated from pure enantiomers are more stable than dimer generated from racemate. This finding indicates that in dimers the interactions between aromatic substituents take place. This observation can be used to differentiate between pure enantiomer and racemate.