Bentonite originated from Macji Do, Serbia, was submitted to acid treatment at 70 degrees C for 30 min, while only the concentration of applied HCl varied. The obtained acid treated samples were used to modify glassy carbon (GC) electrode. The effect of applied acid treatment on the electrochemical behavior of GC electrodes modified with these materials was investigated. Furthermore, the effect of the introduction of nickel into acid treated samples was studied. The incorporation of nickel into acid treated bentonite was achieved by either ion exchange or impregnation/decomposition method. The obtained samples were characterized using the following methods: inductively coupled plasma (ICP), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and electron spin resonance (ESR) spectroscopy. The electrochemical behavior of these samples was tested by cyclic voltammetry in 0.1 mol dm(-3) H2SO4 solution. The ICP, FUR and ESR results exhibited a slight decrease of iron content in the acid treated samples. XRD and FTIR results confirmed that the conditions applied for the acid treatment were mild enough for the smectite structure to be preserved. The electrocatalytic test showed that the current response of Fe2+/Fe3+ oxidation/reduction process increased on the GC electrodes separately modified with each of the acid treated samples in comparison with current obtained on the GC electrode modified with untreated sample. These results indicated that applied acid treatment probably increased the accessibility of the electroactive iron within smectite. Cyclic voltammograms obtained for the GC electrodes modified with acid treated bentonite materials showed greater anodic charge (q(a)) than cathodic charge (q(c)). This difference might be due to iron detachment from smectite structure during the oxidation process. Further modification of the selected acid treated sample with nickel species resulted in decreased current response of the Fe2+/Fe3+ oxidation/reduction process and improved the reversibility of this process. (C) 2015 Elsevier B.V. All rights reserved.