The fate of organic chelating agents in the aquatic environment may be mostly determined by reactions, which regulate the distribution of chemicals between the solid and liquid phase. Especially adsorption/desorption processes are important for the adjustment of the distribution equilibrium. To assess the influence of such processes on the environmental fate of the phosphate substituting chelates 1-hydroxyethane-1,1-diphosphonic acid (HEDP) and nitrilotriacetic acid (NTA), adsorption studies were carried out in aquatic model systems, containing different adsorbents like clay minerals, sediments and sewage sludges.