Calixarene molecules are very powerful ligands for ions and small molecules and for this reason they have been studied with several techniques as models for host-guest systems. In this paper we report the results of a study on the complexation and structural properties of p-tert-butylcalix[6]arene (CAL6) in monolayer at the water-air interface as a function of temperature and subphase composition. The monolayer properties of CAL6 films are fully studied by recording the pi/A spreading isotherms and surface potentials and by calculating the related parameters [limiting area (A(o)), compressibility modulus (C-s(-1)), dipole moment (mu cos theta)]. Changing the temperature (between 20 and 35 degrees C) and the composition of the aqueous subphase (NaCl, KCl, CsCl, guanidinium thiocyanate) produces strong variations on the monolayer parameters, indicating a different selectivity of the CAL6 ligand for the different monovalent cations. In particular, high selectivity in the case of cesium is found. Surface potential measurements as well as limiting area values, compared to CPK model calculations, are discussed in relation to the orientation of the cone-shaped molecules at the water-air interface.