In the present investigation, Aspergillus niger isolated from pistachio shelf was applied to remove iron impurities from an Iranian kaolin sample. In order to study the effects of initial pH, sucrose and spore concentration on oxalic and citric acid production, and consequently iron dissolution, response surface methodology based on a five-level, three-variable central composite design of experiments was employed. Three models were suggested to predict response values based on the mentioned variables. The most important variables on iron dissolution were initial pH, sucrose and spore concentration, respectively. Also, the highest iron concentration, 311.30 mg/l, was obtained when initial pH was 2, sucrose concentration, 70 g/l, and spore concentration, 35 x 10(7) spore/1, and represented the removal of 67.4% of the total iron contents of the clay. (C) 2008 Elsevier B.V. All rights reserved.