With a new cell design, simultaneous electrochemical–ESR–conductivity measurements were realized for the first time. A detailed picture of the potential dependences of the ESR signal and conductance of polyaniline was thus obtained, suggesting that the conduction mechanism could be well interpreted in the framework of the polaron lattice model. Besides, the ESR saturation data obtained in this work provided new experimental evidence for the transformation between Curie and Pauli spins with potential change.