The effect of the acid activation on the efficiency of clay modified electrodes has been investigated. The number of surface-bound [Fe(bpy)(3)](2+), SO42- ion pairs was significantly increased in the acid activated montmorillonite film. Enhanced electroactivity towards [Fe(CN)(6)](3-) anions but no special changes for the electrochemical behavior of H(2)Q neutral molecules were observed. This behavior is attributed to differences in clay morphology, layer charge, and surface area. The role of the clay acidity is also discussed.