A study on the thermal stability, thermodynamical parameters, especially relaxation transition temperature (T-g), and their change upon irreversible reactions (crosslinking, postpolymerization, decomposition, etc.) taking place on heating of the undoped homo- and copolymers of aniline and o-methoxyaniline was performed by using differential scanning calorimetry. It was found that polyaniline and its oligomers might be responsible for the postpolymerization process observed on heating thermograms of the polymers in their emeraldine base form. It can be stated that the proceeding of this process appears to be a reason for the higher thermal stability of polyaniline when compared with poly(o-methoxyaniline). (c) 2007 Wiley Periodicals, Inc.