The adsorption of Congo red onto Ceratophyllum demersum (C. denzersum) has been investigated. It is observed from the experiments that an adsorbent prepared from the Ceratophyllum demersum plant is effective for the removal of Congo red from aqueous solutions at room temperature. The adsorption of Congo red is found to be dependent on contact time, initial dye concentration and adsorbent dose. The results indicate that Ceratophyllum demersum has a strong capability of removing Congo red dye directly from contaminated water. Therefore, it can be concluded that Ceratophyllum demersum could be used for the removal of pollutants from aqueous solutions. The equilibrium adsorption data is analyzed using three common adsorption models: Freundlich, Langmuir and Temkin. The thermodynamic parameters Delta G, Delta H, and Delta S are calculated for the adsorption of Congo red onto Ceratophyllum demersum. The Gibbs free energy change values show that the adsorption of Congo red onto Ceratophyllum demersum is spontaneous.