The reduction of ferrofluid (FF) by L-ascorbic acid (H(2)A) and the reoxidation of the reduced FF to its original form by hydrogen peroxide have been investigated in aqueous acidic medium. The rate of reduction of FF was found to be first order with respect to [H2A] and [FF] and independent with respect to [H2O2] and ionic strength. The rate of reduction of FF increased with increasing pH (2.5-4.0), having an inverse first order dependence in [H+]. With increasing temperature (15-45 degreesC), the rate of reduction was increased in line with the Arrhenius equation. Based on experimental evidence and results a mechanism, operative to reduce FF and reoxidize the reduced FF by H2O2, which makes the system catalytic, is suggested. Thermodynamic quantities associated with FF-catalyzed oxidation of H(2)A by H2O2 were determined and compared with other closely related systems. (C) 2003 Elsevier Inc. All rights reserved.