The formation constant (K-f) for the uranyl complex of 2,2'-dihydroxyazobenzene (DHAB) was measured with DHAB attached to poly(ethylenimine) (DHAB-PEI) at pH 7.7 to 9.4. The value of K-f was estimated from the equilibrium constant for extraction of uranyl ion from the uranyl complex of DHAB-PEI (UO(2)DHAB-PEI) with carbonate ion, which in turn was measured from the absorbance change observed on addition of bicarbonate-ion to the solution of UO(2)DHAB-PEI. At pH 8.0, the uranyl-binding ability of DHAB was enhanced by about 10(4) times on attachment of DHAB to PEI. The major origin of the increased ability of uranyl ion complexation is the basic local microenvironment of PEI, which encourages ionization of the phenol groups of DHAB. Various other possible origins are discussed also. The log K-f for DHAB-PEI at pH 8.0 indicates that DHAB moieties of DHAB-PEI are mostly occupied, whereas DHAB unattached to-PEI is mostly unoccupied by uranyl ion under conditions of seawater when only the pH and concentrations of bicarbonate and uranyl ions of seawater are considered.