The catalytic effect of polyaniline (PANI) on the hydroquinone/quinone (H(2)Q/Q) redox reaction, has been investigated with respect to different morphologies which are known to depend on the counter-ions during the synthesis. It has been established that PANI lowers the H(2)Q/Q reaction potential, as well as the potential difference, DeltaE(p) obtained by cyclic voltammetry It is shown that the decrease in DeltaE(p) does not depend on the increase in the thickness of the PANI layer, and there is no difference in the case of PANISO(4) and PANICI electrode. The increase in the I-p values in the cyclic voltammograms of PANI, as well as in I-lim in rotating disc electrodes (RDE) measurements, compared to Pt-electrodes, is explained by the increase in the real surface area of PANI electrodes, and the difference in PANISO(4) and PANICI by the different morphologies. In general, the redox reaction on PANI electrodes is explained by the specific adsorption of H,Q molecules onto EM2+ centres. The inclined current plateau obtained by RDE measurements is explained by the slow process in the recombination of the reduced PANI (LE) to the oxidised form (EM2+) which in the presence of high concentrations of H(2)Q becomes the rate-determining step in the overall reaction.