In situ ESR-electrochemical measurements indicate that the distinct redox properties of polyaniline synthesized using the interface polymerization method (labeled IP-polyaniline) are strongly related to its unpaired spin density. IP-polyaniline in a 1.0 M NaCl solution of pH 5.5 still holds stronger ESR signals at a wide potential range, which results in its high redox activity in this solution. The influence of pH on the potential range for the formation of polaron is detected. Also, some unusual phenomena are observed in the measurements of ESR signal intensity as a function of applied potential, for example, the ESR signal intensity of IP-polyaniline in 0.20 M HCl solution decreases with increasing potential from 0.30 to 0.80 V accompanied with the peak-to-peak line width Delta H-pp of the ESR signal increasing from 0.30 to 0.60 V, and then, however, Delta H-pp decreases pronouncedly as the potential increases further. The results from measurements for the ESR susceptibility of IP-polyaniline as a function of temperature demonstrate the presence of the conversion of the temperature-dependent Curie susceptibility to the temperature-independent Pauli susceptibility at the temperature range 135-335 K; however, the ESR susceptibility of IP-polyaniline increases again from 335 to 375 K. The Delta H-pp value increases very obviously from 135 to 195 K and then decreases with increasing temperature up to 375 K.