In this study, the effects of several parameters such as the acid concentration, mixing time, temperature, solid rate, and mixing rate on the specific surface area (BET) and cation exchange capacity (CEC) of sepiolite activated by citric acid were investigated using factorial experimental design and response surface methodology. While the specific surface area and cation exchange capacity of nitric acid activated sepiolite were chosen as the responses, the acid concentration, time, temperature, solid rate, and mixing rate were the process parameters. The relationship between the process parameters and the responses was obtained and used in the determination of the optimal conditions. The result showed that the specific surface area and cation exchange capacity of the acid-activated sepiolite were strongly affected by the process parameters. The optimal conditions for achieving the maximum specific surface area and minimum cation exchange capacity were 95.73°C, 8.64%, 4.99 N, 1 h, and 450 rpm for temperature, solid rate, acid concentration, activation time, and the mixing rate, respectively. The experimental and predicted values for the S BET and the CEC were obtained as 450.97 m 2 /g and 449.97 m 2 /g, and 6.79 meq/100 g and 6.86 meq/100 g, respectively. The results obtained from this study clearly indicated that the actual values were in good conformity with the predicted values as evidenced by small errors of 1.00% and 0.07% for the S BET and CEC, respectively. As a result, the S BET and CEC of the Sep activated by citric acid were strongly affected by the process parameters, especially the solid rate for the S BET and the temperature for CEC. It was found that citric-acid activated Sep had a S BET approximately 2.5 times higher than that of the untreated Sep while the CEC of Sep may be strongly eliminated by citric acid treatment. Thus, the mineral structure was progressively transformed into amorphous silica coming from the tetrahedral sheet of the generated fibrous silicates, whose S BET was larger than that of the raw Sep.