Adsorption of Pb(II), Cu(II), Zn(II), Co(II), and Mn(II) to kaolinite was measured at 25 degrees C in the presence of 5 mM KNO3. Adsorption edges (pH 3-10; total metal concentration 100 mu M) for Pb(II) and Cu(II) were sigmoid, but those for Zn(II) and Co(II), and especially Mn(II), were characterized by distinct steps after about 40% adsorption. Adsorption isotherms (concentrations up to 60 mu M; fixed pH) at pH 5.50 followed the simple Langmuir equation, but those at pH 7.50 (Zn(II), Co(II), and Mn(II)) required a two-site model. More protons were released during adsorption at the higher pH. All adsorption data, and the results of potentiometric titrations of kaolinite suspensions (alone and in the presence of metals at 100 mu M), can be fitted closely by a constant-capacitance surface complexation model that incorporates two bidentate surface complexes. One involves electrostatic attraction between transition metal ions and the permanent, negatively charged sites on the silanol faces of kaolinite. The other is an inner-sphere complex at the variable-charge surface hydroxyl groups situated at the crystal edges and on the aluminol faces.