The first steps of the polymerization of phenolic aldehydes (coumaraldehyde (1H), coniferaldehyde (2H), and sinapaldehyde (3H) have been examined by cyclic voltammetry and pulse radiolysis in aqueous (water/methanol mixture) and in organic solvents (acetonitrile and CH2Cl2). The mechanism involves an electron transfer of the corresponding phenolate to give the phenoxy radical which dimerizes. The values of the dimerization rate constants have been measured in water and acetonitrile (ACN) and found to be sensitive to the nature of the solvent. The standard potentials for the different phenoxyl radical/phenolate couples have been determined in the different solvents.