Selective binding of Cu(II), Ni(II), and Zn(II) from aqueous solutions of transition metal salts and their mixtures by Langmuir monolayers of dicetyl cyclen (DCC) was investigated. A direct relationship between the subphase pH and the selectivity of macrocyclic molecules with respect to particular metal ions was deduced from analysis of the pi-A isotherms of DCC monolayers and the X-ray fluorescent spectra of the corresponding Langmuir-Blodgett (LB) films. Unusual inversion of the macrocycle selectivity from preferential binding of Cu2+ to complex formation with Ni2+ upon a decrease in subphase pH was revealed for a DCC monolayer. (C) 2004 Elsevier B.V. All rights reserved.