This work presents the adsorption studies of Cd(II) and Pb(II) ions onto NiO nanoparticles from aqueous solutions. Experiments were conducted under batch process at different time, concentrations and temperature of the suspension. The initial pH of the solution is found to play a crucial role in the process of adsorption of metal ions onto the NiO nanoparticles. Equilibrium experiments shows that the selectivity of NiO nanoparticles towards Pb(II) ions is more than that of Cd(II) ions, which is related to their hydrated ionic radius and first hydrolysis equilibrium constant. Langmuir adsorption isotherm fitted well into the experimental data with high correlation coefficients. Kinetic data revealed that the adsorption is controlled by the film diffusion and experimental data were well fitted by the pseudo-second-order model. The heat of adsorption (Delta H) indicates the endothermic nature of the process and negative values of free energy (Delta G) indicated the spontaneity of the process. EDX analyses provided the evidence for the adsorption of Cd(II) and Pb(II) ions onto the surface of NiO nanoparticles. (C) 2012 Elsevier B.V. All rights reserved.