An in situ electron spin resonance (ESR)-electrochemical method was employed to study the interconversion of polarons and bipolarons of polyaniline during the electrochemical polymerization of aniline. The ESR signal was hardly observed for the first scan during electrolysis of the solution containing 20 mM aniline and 1 M HCl when the potential was swept from 0 to 1.07 V (VS Saturated calomel electrode). However, an ESR signal with a single line was clearly observed when the potential was swept to about 0.95 V for the first scan during electrolysis of the solution containing 0.2 M aniline and 1 M HCl. For the second scan, from 0 V in the positive direction at the same potential range, the ESR intensity first increased and then a peak of the ESR signal, i.e. a maximum value of the ESR intensity, occurred at 0.26 V. This was caused by polyaniline polymerized on platinum due to the first scan; over the peak, the intensity of the ESR signal decreased quickly with increasing potential. The change in the ESR intensity with potential shows that a transfer of polarons in polyaniline to bipolarons occurs from 0.26 to 0.95 V. As the potential increased further, the intensity of the ESR signal increased again from 0.95 to 1.07 V, although this increment is small compared with the intensity change from 0 to 0.26 V. This is caused by the oxidation of aniline, since the intensity of the ESR signal of polyaniline in 1 M HCl solution in the absence of aniline always decreased with increasing potential from 0.26 to 1.07 V. After stopping electrolysis at 1.07 V, the intensity of the ESR signal increased quickly with decreasing potential of polyaniline and then increased slowly when the potential of polyaniline reached an equilibrium value. This indicates that bipolarons in polyaniline immediately transferred to polarons, followed by the establishment of an equilibrium between polarons and bipolarons. The potential range for formation of the ESR signal in the first scan during electrolysis of aniline is extended in the presence of NaCl.