The chemical oxidation of aniline to polyaniline has been monitored by colour and acidity changes during dispersion polymerization. A small amount of various p-phenylenediamines or benzidine was found to accelerate the formation of polyaniline. The ability of such compounds to form cation radicals upon oxidation allows them to act as mediators in electron-transfer processes. Related chemicals, e.g. o-phenylenediamine, had little effect on polymerization, while m-phenylenediamine caused its retardation.