Peroxidase-catalyzed polymerization of sulfonated phenol was investigated by H-1 and C-13 NMR spectroscopy. The reaction process was monitored by H-1 NMR spectroscopy with the incremental addition of H2O2. The variation of the dimer concentration shows three different stages during the reaction: proliferation (I), equilibrium (II), and reduction (III) of the dimer. Conversion of most of the monomer is devoted to the formation of small molecular weight species such as dimers at stage I. A dynamic equilibrium exists between the formation and consumption of the dimer in the reaction system at stage II. The likelihood of formation of high molecular weight products increases at stage III. Structural characterizations of the predominant dimer product and the resulted polymer suggest that C-O-C coupling is dominant in the polymerization, leading to the synthesis of a polymeric material with sulfonated phenylene oxide as the major unit.