Loss of electroactive dopants from conducting polymers (CPs) was investigated by electrochemical quartz crystal microgravimetry (EQCM). pPy[ABTS], polypyrrole doped with ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate), was used as a model system. Two different kinds of electrolytes were used for studying the loss of dopants under electrochemical stimuli: one is a solvent to dissolve the dopant (dopant-philic) while the dopant is insoluble in the other (dopant-phobic). Degradation of polypyrrole backbone as well as loss of the dopant from pPy[ABTS] was observed in the dopant-philic electrolyte. Severe chemical overoxidation of polypyrrole by the most oxidized state of ABTS {(ABTS2+)2−} was emphasized as a factor responsible for de-doping in addition to potential-driven overoxidation or ion exchange. In the dopant-phobic electrolyte, however, the chemical degradation of the polymer film was suppressed.