The cathodic reduction of 1,2-naphthoquinone-4-sulfonate (NQS) at a polyaniline-coated ITO glass electrode was studied with in situ UV-vis spectroelectrochemistry. An electrocatalytic mechanism of this process is proposed, which includes the cathodic reduction of the PANI film and its subsequent reoxidation by NQS. Accordingly, PANI acts as a redox mediator cycled between its emeraldine and its leucoemeraldine form. In the equilibrium state, both redox forms of PANI are present in the film. The relative content of both forms was shown to depend on the concentration of NQS in the solution, on the PANI film thickness and on the applied electrode potential. In addition the cathodic reduction of sulfopropylviologen (SPY) and benzylviologen (BV) was studied in the potential range where the non-conducting (reduced) leucoemeraldine form of PANI exists. The slowness of the cathodic reduction of both BV and SPV, proceeding at pH 0.5, may be caused by electrostatic repulsion between the positively charged PANI polymer chain and the viologen molecule. At pH 4.0, the reduction becomes fast, probably due to deprotonation of the leucoemeraldine form of PANI, resulting in the disappearance of the electrostatic repulsion. In the middle pH region (pH 1.5), a large difference in the rate of the cathodic reduction between BV and SPV was observed. This difference may be attributed to deprotonation of the sulfogroups of SPV. Deprotonated SPV has no net electric charge, and no electrostatic repulsion should be effective. In contrast, BV has a positive electric charge independent of the pH-value of the solution.