Abstract: The removal of Cu²⁺ and Cd²⁺ in water by water hyacinth roots was studied. The results show that with individual metal pollution, water hyacinth roots can remove Cu²⁺ and Cd²⁺ efficiently. However, in a dual-metal arrangement, Cu²⁺ was found to pose a strong inhibition effect on Cd²⁺ uptake by the biosorbent. The release of Ca²⁺, Mg²⁺, K⁺ and H⁺ from water hyacinth roots confirms the existence of an ion exchange process in the biosorption of the two cationic metal ions. X-ray photoelectron spectroscopy further reveals that amine and oxygen-containing groups in the biosorbent also contribute to the metal uptake via chelation. Experiment with Cd²⁺-sorbed biosorbent shows that Cu²⁺ can displace almost all of the adsorbed Cd²⁺ from their binding sites on the biosorbent. In a several-metal arrangement, the competitive inhibition is a critical factor causing the different sorption amounts. The mechanism of competitive inhibition was also studied.