A general equation, obtained from the classical TPD theory and statistical thermodynamics, has been designed for determining the enthalpy of adsorption, DeltaH, from a series of temperature-programmed desorption (TPD) runs carried out with various sample masses, heating rates, or inert gas flow rates. This equation applies to TPD performed either under vacuum or under an inert gas stream, provided that readsorption occurs freely during the experiments. It has been applied to ammonia TPD (under vacuum) from HY zeolite samples of different masses. The pressure of ammonia over the sample was in the range 1 x 10(-4)-15 Pa. The value of DeltaH obtained is -136 kJ/mol. Results showed that the molecule of ammonia in the gas phase completely loses its translational motion when it is adsorbed. Finally, a simple experimental method has been proposed to rapidly investigate whether readsorption occurs during the TPD runs. It has been found that, under our experimental conditions, free readsorption effectively takes place at temperatures under 750 K but that desorption is irreversible above this temperature.