Diatomite, an abundant inorganic material in Jordan, was treated with sulfuric acid and heat treated in order to improve its performance as an adsorbent. The acid treated diatomite (ATD) was used to adsorb methylene blue from aqueous solutions. The equilibrium data were fitted to different adsorption isotherms and the best fit was obtained using Langmuir isotherm with R-2 value of about unity. The maximum loading capacity, Q(max) was found 126.6 mg/g at 30 degrees C and increases slightly as the temperature increases. The kinetics data were best described by the pseudo-second order model with a value of the model constant k(2) of 1.056 (g/mg h) at 30 degrees C and an initial dye concentration of 50 mg/l. The thermodynamic parameters such as the standard enthalpy (Delta H-0), standard entropy (Delta S-0) and standard free energy (Delta G(0)) were evaluated. The obtained results indicated the adsorption of basic dyes onto ATD is endothermic and spontaneous process and confirmed the applicability of this modified inorganic material as an efficient adsorbent for basic dyes.