This study is on the kinetics and thermodynamics of the adsorption of Pb 2, onto phosphate-modified and unmodified kaolinite clay obtained from Ubulu-Ukwu in Delta State of Nigeria. Increasing initial Pb2+ concentration increased the rate of Pb2+ adsorbed with increase in initial Pb2+ concentration from 300 to 1000 mg/L. Increasing Pb2+ concentration also increased the initial sorption rate h, from 1.404 to 13.11 mg g(-1) min(-1) for phosphate-modified kaolinite clay and 1.04-3.48 for unmodified kaolinite clay as Ph 21 concentration. Increase in temperature was found to increase the initial sorption rate of Pb2+ adsorption onto phosphate-modified adsorbent from 3.940 to 8.85 and 2.55 to 4.16 mg g(-1) min(-1) for the unmodified adsorbent. The overall sorption rate k, increased only slightly from 5.1 x 10(-2) to 9.7 x 10(-2) g mg(-1) min(-1) for phosphate-modified adsorbent, 3.8 x 10(-2) to 5.4 x 10(-1) g mg(-1) min(-1) for unmodified adsorbent. The adsorption reaction on both adsorbents was found to be chemically activated reaction and endothermic with energy of activation, E, at 500 mg/L of Pb2+ in solution as 19 and 10.68 kJ mol(-1) for phosphate-modified and unmodified adsorbents, respectively. The positive values of both Aff and AS' obtained suggest an endothermic reaction and in increase in randomness at the solid-liquid interface during the adsorption of Pb 21 onto the adsorbents. AG degrees values obtained were all negative indicating a spontaneous adsorption process. The presence of Cd2+ decreased both initial sorption rate and the amount of Pb2+ adsorbed on phosphate-modified and unmodified adsorbents at equilibrium. The adsorption process follows a pseudo-second-order reaction scheme. (C) 2006 Published by Elsevier B.V.