The anomalous responses of M w to conversion and to comonomer stoichiometry, i.e., deviations from expected classic A−A + B−B step-growth polycondensation behavior, observed in poly(p-phenylene sulfide) synthesis from p-dichlorobenzene and sodium sulfide are shown to be a consequence of unequal rate constants for the growth steps. The polymerization is simulated by a computer model based on the step-growth polymerization concept and includes cyclization and side reactions. Rate constants and activation energies for the growth and side reactions were obtained by measurements of reaction rates of actual reaction intermediates or model compounds, while those for cyclizations were estimated.