The adsorption of aqueous Pb(II), EDTA, and Pb(II)-EDTA complexes onto TiO2 were studied at both stoichiometric and nonstoichiometric Pb(II)/EDTA concentrations. For Pb(II)-TiO2 and EDTA-TiO2, a typical cationic and anionic-type of adsorption was noted, respectively. For 10(-3) and 10(-4) M Pb(II)-EDTA systems, near-equal adsorption of Pb(II) and EDTA indicated that the complex adsorbs as a single species. Also, a ligand-type Pb(IT)EDTA adsorption, i.e., decreasing adsorption with an increase in the pH, was noted. Systems with EDTA greater than Pb(LI) showed near-zero lead removal; competitive adsorption of EDTA and Pb(II)-EDTA onto TiO2 was suggested to cause this effect. For Pb(LI) concentrations (5 x 10(-4) and 10(-3) M) higher than EDTA (10(-4) M), significantly higher EDTA adsorption at high pH as compared to individual 10(-4) M EDTA and 10(-4) M Pb(II)EDTA systems was noted. Adsorption modeling was completed employing the geochemical speciation model MINTEQA2 employing the diffuse layer model. Inner-sphere complexation was considered to occur between Pb(LI), EDTA, Pb(LI)-EDTA, and the TiO2 surface sites. Surface complexes used in the modeling included Ti-O-Pb+, Ti-EDTAH(2-), Ti-EDTA-Pb-, and Ti-O-Pb-EDTA(3-). The cationic-type complexation, Ti-O-Pb-EDTA(3-), was postulated to explain and model the anomalous EDTA adsorption as noted for Pb(II) > EDTA studies. Results from the present study show that the adsorption behavior in aqueous metal/EDTA systems will change with any variation in the contaminant concentration ratios.