The influence of free and total concentrations of Cd²⁺ in the presence and absence of EDTA on the accumulation in Spirodela polyrhiza (L.) Schleiden, strain SP₂₀, and Lemna aequinoctialis Welwitsch, strain LP₆, was investigated. Accumulation by both plant species correlated with the concentration of free Cd²⁺ rather than with total concentration. However, when the ion was removed by the plants from the solution, the EDTA-Cd²⁺ complex provided additional free Cd²⁺ and accumulation was higher than expected from the calculated free Cd²⁺ pool. Because of the high complex stability constant of Fe^III-EDTA, Fe³⁺ competed with Cd²⁺ for EDTA in the solution and, thus, cadmium accumulation was increased by the presence of Fe³⁺. Only after reduction of Fe³⁺ to Fe²⁺ (low complex stability) or iron uptake by the plants, could the additional EDTA complex Cd²⁺ in solution and inhibit its accumulation.