We present the new host mol. (I) which binds basic amino acid esters in water. It recognizes both pos. charged groups of the amino acid esters by electrostatic and hydrogen bond interactions with its four strategically placed phosphonate anions. Selectivity for lysine is achieved by the correct distance between both bisphosphonate pairs. By contrast, the smaller amino acid esters arginine, ornithine, and histidine form 2:1 complexes with I. In methanol, a double chelate assembly enforced by p-cation interactions with the imidazolium cation leads to a very high assocn. const. for the 1:histidine complex of 3 * 104 M-1.