Poly(N-methyl aniline) (PMAn) can be doped and dedoped reversiblly by the treatment of protonic acid and base. The conductivity

which depends on the doping level

increases seven orders of magnitude after doping

but can not reach as high as that of the as-made doped PMAn. The doping process

in nature

is only the protonation of the polymer chain and independent of the counter ions. The doping levels are related to the equilibrium pH values. FTIR

UV-Visible and ESR spectra were employed to study the doping process and the structure of doped PMAn. The results revea that the chain structure cahanges with doping

and a lot of delocalized unpaired electrons exist in the doped PMAn

which favour the conductivity It seems that the free carrier of the electric conduction of PMAn is the delocalized cationic free radicals.