p-tert-Butylcalix[6] arene, CAL6, has been functionalized by the introduction of amide moieties (OCH2-CONEt(2)) on the six phenolic OH groups to give the corresponding hexamide, HXAM, which shows an interesting selectivity toward the guanidinium cation. The present study reports the behavior of HXAM monolayers at the water-air interface as a function of temperature and subphase composition. The parameters obtained from the spreading isotherms (limiting areas, A(0), compressibility moduli, C-s(-1) = (IIA)(partial derivative pi/partial derivative A), and changes in the Gibbs free energy upon passing from water to salt subphase, Delta G(salt-water)) and the surface potential data show that the HXAM assumes a "cone-shaped" conformation. The molecules are in a parallel orientation at the water-air interface with the six amide groups in the water phase. Changes in the subphase from water to salt solutions (NaCl, KCl, CsCl, guanidinium thiocyanate) affect the spreading isotherms but not the surface potential behavior. The results are discussed in terms of the two-dimensional phases of the monomolecular films, the stoichiometry of the complexes with the cations, and the conformations of uncomplexed and complexed HXAM molecules. The comparison of these results with those of the unsubstituted CAL6 behavior shows the high selectivity of HXAM monolayers toward the guanidinium cation.