Triphosphonate cavitand, 3iPO, has been designed for the recognition of ammonium in solution. This Letter presents a study of the selective guest-host recognition between Acetylcholine and Choline ions, ACh(+) and Ch(+), and the host. We are able to observe that the ACh(+) ion replaces the Ch(+) ion in the 3iPO-Ch(+) complexes with a concomitant exchange/substitution constant of 7 x 10(4) M-1. Conversely a replacement of the ACh(+) ions by the Ch(+) ions in the 3iPO-ACh(+) complexes has been also observed with a concomitant exchange constant of 5.8 x 10(3) M-1. The structure of the 3iPO-ACh(+) complex is determined by single crystal X-ray crystallography. (C) 2012 Elsevier B. V. All rights reserved.