gamma -Al2O3 samples, both pure and Cr doped, were prepared by heating at 470 degreesC sol-gel precursors obtained by using aluminum tri-sec-butoxide [Al(OC4H9)(3)] as the starting compound. The samples were characterised for phase composition (X-ray diffraction) and surface area (BET). Electrochemical determinations of the double layer reactivity of the purr and doped oxides were performed both by surface charge and by electrokinetic sonic amplitude (ESA) determinations. The interfacial electrostatic response is discussed and analysed also with reference to the sample surface state obtained by X-ray photoelectron spectroscopy (XPS).