In this study, raw diatomite was purified by thermal treatment in order to improve the adsorption capacity of diatomite. The prepared calcined diatomite samples were characterized by the X-ray diffraction (XRD), scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis techniques. The raw and thermally modified diatomite at 500 degrees C was tested for the adsorption of Pb(II), Cu(II) and Cd(II) from aqueous solutions. Adsorption experiments were performed under batch process, using metal ions initial concentration, contact time and temperature as variables. The linear Langmuir, Freundlich and Dubinin-Radushkevich (D-R) adsorption equations were applied to describe the equilibrium isotherms. Equilibrium studies showed that thermally modified diatomite has a higher removal capacity for Pb(II), Cu(II) and Cd(II) from water than untreated diatomite. The kinetic data were evaluated using the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic equations. The experimental data proved a closer fit to the pseudo-second-order model. Thermodynamic parameters such as the enthalpy (Delta H-0), Gibbs' free energy (Delta G(0)) and entropy (Delta S-0) were calculated for raw and thermally modified diatomite. These values showed that the adsorption of Pb(II), Cu(II) and Cd(II) ions onto diatomite samples was controlled by a physical mechanism and occurred spontaneously.