The processes governing adsorption of anthropogenic organic molecules on natural systems are complex. The presence of various phases in soils and mudstones may lead to various and simultaneous retention mechanisms. Adsorption experiments were performed on acetic, o-phthalic and citric acids on the Callovo-Oxfordian clay rock. Whereas the retention of inorganic species is dominated by clay minerals, small carboxylic acids were found to sorb on various phases or sites and not necessarily endured competition during co-adsorption. Data will be presented to illustrate the interest of various adsorption protocols to discriminate between mechanisms occurring beside adsorption onto soils and sediments covalent bounding, chemical perturbation, phase dissolution, bacterial activity, etc...