Sodium poly(aspartic acid) (PASP) was investigated for application in the diffusive gradients in thin films (DGT) technique as a new binding phase (PASP DGT) for the measurement of Cd2+ in waters. The Cd2+ uptake of PASP DGT increased linearly with time (r(2) = 0.9989), with a DGT-labile fraction (Psi) of 97.57 +/- 1.45% in synthetic solution. The Psi values of Cd2+ were measured in spiked river water (48.65 +/- 2.26%) and spiked domestic wastewater (26.71 +/- 1.25%). The binding capacity of PASP DGT decreased with increasing ionic strength, whereas it remained fairly constant in the pH range 4-10. The stability constant (log K) of PASP with Cd2+ was 9.28, indicating that PASP can coordinate strongly with Cd2+. The results show that PASP can be used as a binding phase in liquid-type DGT devices for the measurement of Cd2+ in waters.