Gas-phase proton-bound complexes between a chiral resorcin[4]arene and some representative amino acids, that is, l- and d-alanine or l- and d-serine, were generated in the source of a Fourier transform ion cyclotron resonance mass spectrometer. Gas-phase exchange of the amino acid from the diastereomeric complexes with the enantiomers of 2-butylamine exhibits a significant enantioselectivity, which depends not only upon the configuration of the leaving guest but also on that of the incoming amine. These findings, coupled with molecular dynamic calculations, point to the observed gas-phase enantioselectivity as determined by the effects of the resorcin[4]arene chiral cavity upon the diastereomeric exchange transition structures. Copyright © 2002 American Chemical Society.