The thermal degradation/stability of poly(o-phenitidine) doped with different sulfonic acids was investigated with the help of thermogravimetric analysis coupled with mass spectrometer. Thermal decomposition behaviour was found to depend on the nature of dopant ion and the thermal stability was in the order of DBSA > CSA > PTSA. The dopant ion was found to dissociate prior to the main chain scission. These findings could be explained on the basis of strong coupling/complexation between the conducting polymer and the dopant as well as the stability of the individual dopant species. © 2001 Elsevier Science Ltd.