Reactor design for the ferrous-catalyzed oxidation (FCO) of 2,4,6-trichlorophenol (TCP) was developed through an examination of the reaction kinetics under various conditions. The TCP decay kinetics was found to consist of reaction and stagnant stages, and two characteristic constants, the initial decay rate and the oxidative capacity of the FCO, were successfully derived and used to describe the respective stages. The proposed model and the method of solving the two characteristic constants is capable of describing the TCP degradation precisely in FCO at different Fe(II) concentrations and Fe(II)/H2O2 concentration ratios, where an Fe(II)/TCP concentration ratio of over 1 was found to be the critical point to achieve 100% removal of TCP by FCO. Equations for practical reactor sizing for predetermined performance were also developed on the basis of the two characteristic constants.