The surface characterization of MgY and NH4Y zeolites was performed using inverse gas chromatography (IGC). The adsorption thermodynamic parameters (the standard enthalpy (Delta H degrees), standard entropy change (AS'), and free energy change of adsorption (Delta G degrees), the dispersive component of the surface free energies (gamma(d)(s)), and the acid-base character of the surface of MgY and NH4Y zeolites were estimated using the retention time of different non-polar and polar probes at infinite dilution region. The specific free energy of adsorption (Delta G(sp)), the specific enthalpy of adsorption (Delta H-sp), and the specific entropy of adsorption (Delta S-sp) of polar probes on MgY and NH4Y zeolites were determined. The values of the Delta H-sp were correlated with both the donor and acceptor numbers of the probes to quantify the acidic K-A and the basic K-D parameters of the zeolite surfaces. The values obtained for the KA and KD parameters indicated a basic character for the surface of MgY and NH4Y zeolites. (c) 2007 Elsevier B.V. All rights reserved.