BACKGROUND: Serbian smectite clay has been activated using a response surface method with central composite design. Because, among the criteria of an edible oil, colour is the most important factor in the commercial value of the oil, and since the colour is due to the presence of pigments such as beta-carotene in the crude oil, acid-activated clay was used for decolourisation of soybean oil. RESULTS: The effects of five parameters, namely, temperature, time, acid strength, solid/liquid ratio and stirring speed, on the process of acid activation of the clay and its bleaching capacity were determined using a statistical model. The results indicated that all parameters were significant factors in the bleaching capacity of acid-activated clay, and a quadratic polynomial equation for bleaching capacity was obtained by multiple regression analysis. The optimal bleaching capacity was estimated to be 96.61% for an experimental run under the following conditions: activation temperature 80 degrees C, HCl concentration 4.78 mol L-1, stirring speed 450 rpm, solid/liquid ratio 1:4.5 and activation time 2.81 h. CONCLUSIONS: The central composite design, regression analysis and response surface method were effective in identifying the optimal conditions for bleaching capacity of acid-activated smectite. (C) 2008 Society of Chemical Industry