A simple model is presented for the polymerization of poly(phenylene oxide) (PPO) in the presence of mass-transfer limitations. The mass balance and moment equations for this system compose a hierarchy of infinite ordinary differential equations. Closure conditions similar to those used for nylon 6 are used to truncate the set of moment equations. The rate constants are adjusted to achieve a reasonable fit between model predictions and one set of available experimental data on the variation of the number-average chain length (mu(n)) with time. The monomer conversion and the polydispersity index (PDI) are predicted. The PDI of the entire macromolecular species is found to vary with time in an interesting manner (a decrease is predicted with time during certain periods). This is due to the effect of mixing of two sets of macromolecular species present in the reaction mass having different concentrations, as well as different individual values of mu(n) and PDI. Sensitivity results are reported to identify the important parameters and operating conditions. It is found that the values of mu(n) are very sensitive to even small changes in the operating conditions which decrease the mass-transfer rates.