In this paper the adsorption process of a natural steroid hormone (progesterone) by a carbon black and a commercial activated carbon has been studied. The corresponding equilibrium isotherms have been analyzed according to a previously proposed model which establishes a kinetic law satisfactorily fitting the C versus t isotherms. The analysis of the experimental data points out the existence of two well-defined sections in the equilibrium isotherms. A general equation including these two processes has been proposed, the global adsorption process being fitted to such equation. From the values of the kinetic equilibrium constant so obtained, values of standard average adsorption enthalpy (Delta H degrees) and entropy (Delta S degrees) have been calculated. Finally, information related to variations of differential adsorption enthalpy (Delta H) and entropy (Delta S) with the surface coverage fraction (theta) was obtained by using the corresponding Clausius-Clapeyron equations. (c) 2006 Elsevier B.V All rights reserved.