Abstract In situ EPR and electrochemical experiments of polyaniline films were carried out in tetrafluoroboric, perchloric, sulphuric and hydrochloric acid solutions. Experiments were done by following the number of spins during cyclic voltammetric scanning and by recording the EPR spectra at constant potential. The polyaniline films were prepared in HBF 4, and cycled in the same acid or transferred to other acid solutions to be cycled. The differences in the EPR spectra were interpreted using the theory described by Epstein et al. concerning the Curie and Pauli spins and islands of high conductive polymer. At the beginning of cycling, the spins are supposed to be Curie spins, which appear first with a broad signal ( H=10.8 G), then change to a narrow signal ( H = 0.5 G) after they are partially transformed to other narrow signals supposed to be Pauli spins with a g value slightly lower and very close to that of the free electron. ClO 4 − and Cl − anions are the more efficient anions for forming Pauli spins, BF 4 − is less effective and sulphate or hydrogen sulphate give only Curie spins. The influence of ions on the nature of the spins must be interpreted in the light of ionic interactions between the positive charges of the solvated polymer chain and the solvated anions of the solution.