Polyaniline is prepared by chemical polymerization of aniline in an acidic solution using H2O2 as an oxidant and ferrous chloride as a catalyst. A wide variety of synthesis parameters are studied, such as the amount of the catalyst, reaction temperature, reaction time, initial molar ratio of oxidant, monomer and catalyst, and aniline and HCl concentrations. The polymerization of aniline can be initiated by a very small amount of catalyst. The yield and the conductivity of product depend on the initial molar ratio of the oxidant and monomer. The polyaniline with a conductivity of about 10 degrees S/cm and a yield of 60% is prepared under optimum conditions. The process of polymerization was studied by in situ ultraviolet-visible spectroscopy and open-circuit potential technology. Compared to the polymerization process in a (NH4)(2)S2O8 system, the features of the H2O2-Fe2+ system are pointed out, and the chain growth mechanism is proposed.