The effectiveness factor; E (f) , defined as the fraction of the surface that participates effectively in a given reaction, is an important parameter when operating three-dimensional (3D) electrodes. The rotating disk electrode (RDE) technique with the Fe3+/Fe2+ redox couple as a probe reaction has been used for the evaluation of the effectiveness factor of 3D Ti/IrO2 electrodes with different IrO2 loading. For this purpose, steady-state polarization measurements using Ti/IrO2 rotating disk electrodes in 0.5 M Fe3+/Fe2+ in 1 M HCl were carried out under well-defined hydrodynamic conditions. The low-field approximation relation has been used for the estimation of the exchange current densities j (0), of the Fe3+/Fe2+ redox couple. It was found for this redox couple that the effectiveness factor is very low (< 2%) and essentially the 2D electrode surface area works effectively in the steady-state polarization measurements.