An in situ UV-Vis spectroelectrochemical study of the interaction of some redox species with a polyaniline-modified optically transparent electrode was performed under controlled potential and open circuit conditions. It is shown, that an estimate of the content of both emeraldine (oxidised) and leucoemeraldine (reduced) forms in the polymer film is possible during electrolysis of redox-active species with the polymer deposited on transparent ITO electrodes. The reduction of the emeraldine form by hydroquinone was observed after a positive potential switch, resulting in a diminished fraction of the emeraldine form in the polymer film at a quasiequilibrium state during electrolysis. Also, the oxidation of the leucoemeraldine form by quinone, Fe3+ ions, dissolved dioxygen and hydrogen peroxide was demonstrated to proceed after a negative potential step. The dependence of the relative fraction of both redox forms in the PANI film under conditions of quasiequilibrium on the concentration of redox species was studied. The results obtained are discussed within a framework of two processes taking place in PANI-coated electrodes : the electrochemical reduction or oxidation of the PANI film and the chemical redox interaction of leucoemeraldine or emeraldine forms with oxidising or reducing species in solution, respectively.