Temperature-programmed desorption (TPD) of NH3 has been used to obtain information about the strength of the interactions between ammonia and a Na-Y zeolite. Monte Carlo simulation was employed to interpret the TPD spectra taking into account a discrete inhomogeneity of the zeolite surface. Zeolite showed a significant amount of adsorption sites (80%) distributed in the adsorptive potential range between 22 and 33 kcal/mol. The heat of adsorption decreased with increasing ammonia coverage from 30 to 19.4 kcal/mol, and these values were in agreement with microcalorimetric measurements.