The aim of the study was to determine the highest bleaching capacity and derive a model for acid activation of a Turkish bentonite using a full factorial design. The selected factors were contact time, solid to liquid ratio, acid concentration and moisture of bentonite. A first-order model is obtained by using 2 4 full factorial design. Auxiliary experiments for the second-order model were conducted according to an orthogonal central composite design. An experimental test carried out using a factorial design 2 4 indicated that contact time, solid to liquid ratio and moisture of bentonite have a positive effect, whereas acid concentration has a negative effect. Furthermore, the highest bleaching capacity was found to be 74.2%. (c) 2005 Elsevier Ltd. All rights reserved.