The plant polymer lignin represents one of the most structurally diverse natural products and results from the oxidative coupling of phenylpropanoid monomers. Peroxidase catalyses the oxidation of phenylpropanoids to their phenoxyl radicals, and the subsequent nonenzymatic coupling controls the pattern and extent of polymerisation. Using EPR spectroscopy, we have demonstrated that for a series of substrates increased methoxylation increases peroxidase-catalyzed oxidation and that this is most easily achieved with the monomeric alcohols. Dimeric compounds, synthesized to represent the initial products of phenylpropanoid coupling, showed a marked decrease in their ability to be oxidized when compared with the monomeric substrates. These findings demonstrate that the structure of the monomer determines the final composition of lignin, which ultimately effects the overall structure. The results indicate that the polymer grows primarily as a result of the reactivity of the monomers and that polymerization to high molecular weight may be restricted to methoxylated species.