The removal of methylene blue, reactive black (C-NN), and reactive yellow (MI-2RN) from aqueous solution by calcined and raw diatomite at 980 degrees C was studied. These studies demonstrated the importance of the various functional groups on the mechanism of adsorption. The role of pore size distribution in the dye adsorption studies was also investigated. The adsorption isotherms were pH dependent. Henry and Freundlich adsorption isotherms were used to model the adsorption behavior and experimental results for all dyes used exhibited heterogeneous surface binding. The removal of the ionisable functional groups increased the pH(ZPC) value from 5.4 to 7.7, while FTIR, SEM and XRD analysis showed a remarkable decrease of the characteristic Si-OH peaks after calcinations at 980 degrees C. The removal of hydroxyl groups from the surface of diatomite lead to a decrease in the adsorption. It was evident from pH and infrared spectra results that mechanisms of methylene blue and reactive yellow adsorption differed from that of reactive black. Accordingly, adsorption on the external surface by n-pi interaction between the pi system of the RB and the electron lone pairs of the oxygen atoms of siloxane group and columbic attraction between the dye and the surface of calcined diatomite was proposed as a possible adsorption mechanism.