We present the first study of the chemical interconversion between leucoemeraldine and emeraldine bases of polyaniline in relation to the molecular weight and molecular weight distribution. Electronic absorption spectroscopy and gel-permeation chromatography were employed to monitor the changes in the oxidation state and molecular weight of the polyaniline, soluble in 1-methyl-2-pyrrolidinone, during the redox reactions. The molecular weight and molecular weight distribution of polyaniline were found to change drastically when the redox reactions were performed with the polymers in the solid phase while relatively little when the reactions were carried out with the polymers in the solution phase. The changes observed in the solid-phase reactions are attributed to the chemical cross-linking and/or branching by interchain reactions in the polymer particles, which are favored by the small interchain distances.