China clay is an important mineral, which is used in the manufacture of ceramics and refractory, as also in other industries. Mined China clay contains iron oxides and silicates as impurity ; if present in excess of a threshold level, the impurities affect the commercial value of the products. Currently available processes for lowering the iron content in China clay to the desired level (<0.8%) are energy- and cost-intensive, not sufficiently flexible, and may cause environmental pollution. An alternative approach for iron removal consists in the development of a biotechnological process which is expected to be cost-effective, less complex and eco-friendly. We reported earlier that several fungi, especially Aspergillus niger, and their culture filtrates could leach sufficient amount of iron from a China clay sample; oxalic acid was found to be the most active component of the culture filtrate (Trans. Indian Inst. Met. 55 (2002) 1). We now report the rates of iron leaching from another China clay sample by oxalic acid and by the culture filtrate of A. niger NCIM 548 that was found to be the most active strain in our previous study (Trans. Indian Inst. Met. 55 (2002) 1). The iron-leaching rates increased with temperature (7) and followed biphasic kinetics. The effect of oxalic acid concentration (C), pH (R), solids concentration or pulp density (P), time and mode of agitation on the rate of iron leaching is described. The rate of leaching with oxalic acid (R-ox) can be calculated theoretically from the following relationship: R-ox similar to (C)(0.76)(T)(1.76)(H)(0.80)(P)(0.20) under the specified set of conditions. Using the same concentration of oxalic acid in A. niger culture filtrate, the relationship of the rate differed; this may be due to the influence of other metabolites present in the culture filtrate on the rate. (C) 2004 Elsevier B.V. All rights reserved.