A detailed kinetic model is developed for the polymerization of poly(phenylene oxide) (PPO). The Box complex technique is used to obtain the best-fit values of the parameters of the model (rate constants), using experimental data on the variation of the number-average chain length (mu(n)) with time for one set of conditions. A sensitivity study helps to simplify this kinetic scheme. The Box complex method gives the optimal values of the four rate constants associated with this simplified model. The model is able to explain the two interesting features of this polymerization, namely : (i) a slow increase in mu(n) with time during the initial period, followed by a sharp increase for a short duration; and (ii) a decrease in the polydispersity index at some intermediate time, associated with the mixing of several polymeric species having different individual chain-length distributions. The kinetic model can be adapted for use for simulating industrial PPO reactors wherein additional physical phenomena like heat and mass transfer are present.