The electrocatalytic oxidation of ferrocene (Fc) at neutral pH in aqueous solution has been studied over thin films of electroactive sulfonated polyaniline (SPAN) by cyclic voltammetry. Strong adsorption of the oxidised species (ferrocenium ion) on the polymer arises as the explanation for the negative shift of the oxidation potential. FTIR-in situ studies provide further evidences of ferrocene/ferrocenium (Fc/Fc+) adsorption and the sulfonate moieties in the polymer are proposed as adsorption sites. Adsorption of ferrocenium by cyclic potential sweep is suggested as a suitable method for its implementation into second generation biosensors based on SPAN polymer, where it would play the role of redox mediator. ; Financial supports by the Ministerio de Educación y Ciencia y FEDER (MAT2007-60621) and Generalitat Valenciana (Red Arviv/2007/076).