In situ IR spectroscopic study was made on the electrochemical doping of poly(N-vinylcarbazole) (PVK) by the subtractively normalized interfacial Fourier transform technique. The results show that the doping of ClO4- anion is limited by crosslinking of the polymer. The structure of doped PVK film depends on electrode potential and reaction time. Maximum doping of PVK may be achieved using a low oxidation potential for a proper reaction time.