The redox behaviour of poly( N-vinylcarbazole) (PVCz) films deposited potentiostatically on platinum wire has been studied by means of reversed current pulse chronopotentiometry, spectrophotometry and EPR spectroscopy. The galvanoslatic method was used to determine the surface concentration of the redox centres in PVCz and the formal degree of oxidation of the polymer at various polarisation potentials in acetonitrile solution of supporting electrolyte. The EPR spectra correlated with UV-visible absorption spectra of the oxidised polymer suggest that polarons are the majority charge carriers responsible for conductivity of PVCz. Decrease of polaron concentration in acetonitrile + water mixtures recorded under open circuit condition was interpreted in terms of internal configuration of the polymer backbone. The partial trapping of the polarons in the polymer matrix was also proved by means of EPR at high water content (90%) in the acetonitrile solution.