The presents study brings to light that the free energy reduction per surface unit, Deltag, which is considered as the driven force that leads the imbibition of liquids into porous media, does not show any dynamic behaviour during the rise of the liquids. Therefore, this quantity is a constant parameter that characterized the capillary rise processes. As a consequence, it has been also proved that, because of this fact, the liquid flow verifies the hypotheses that are needed in order to the results of the imbibition experiments can be analysed by Washburn's equation. This study has been carried out by means of a new methodology based on the analysis of the velocity profile associated to the increase in the weight of the porous medium caused by the rise of the liquid. (C) 2004 Elsevier B.V. All rights reserved.