The aims of the present study are to propose a mathematical model for photocatalytic decomposition of 2,4-dinitrophenol (DNP) in an aqueous suspension of TiO2 fine particles and to elucidate the reaction mechanism using this model. The following three facts were found experimentally: (i) although a reactant usually decomposes more quickly at higher concentrations, DNP decomposed more slowly; (ii) photocatalytic reaction usually obeys firstorder kinetics when the initial reactant concentration is approximately 10 g m(-3), although the DNP concentration decreased almost linearly according to zeroorder kinetics; and (iii) the resulting NO3 ion significantly decreased the DNP decomposition rate. A mathematical model constructed by taking into consideration these findings can successfully explain the experimental data, implying that DNP molecules are highly aggregated in the neighborhood of TiO2 particle. The DNP aggregate layer suppresses the generation of active radicals, which leads to reactant inhibition. The photocatalytic decomposition of DNP supplied from the aggregate layer of highly concentrated DNP around TiO2 is observed by zero-order kinetics. (C) 2013 Elsevier B.V. All rights reserved.