The adsorption of Acid blue 25 (AB 25) dye from aqueous medium on diatomite was studied. The effects of pH, contact time, initial dye concentration, calcinations and sorbent dosage on the adsorption process were examined and optimum experimental conditions were identified. The maximum removal of dye was obtained under acidic conditions, in particular, at pH 2, but pH 8 was more suitable for industrial purposes. The percentage removal of dye decreased with an increase in initial concentration. Also, the results indicated that an increase in temperature raised the percentage removal of dye. The maximum percentage removal of AB 25 dye from aqueous solution using raw diatomite at pH 2, temperature 25 +/- 1 degrees C, agitation speed of 200 rpm, initial dye concentration of 50 mg/L, and for a mixing time period of 300 min was 72.81% (64.22% at pH 8). Furthermore, under same conditions, the maximum adsorption of dye on calcined diatomite was 54.5%. The experimental data showed that the adsorption of dye on raw diatomite follows the Langmuir model, but its adsorption on calcined diatomite followed the Freundlich model. The kinetics effect of the adsorbent was pseudo-second-order.