Changes of electrical properties of dry and wet gamma-Al2O3 surface during interaction with a propylene-air mixture was studied in situ, by following a standard protocol of experiments, under simultaneous monitoring of the composition of the inlet/effluent gas. Data were compared with results obtained on a higher surface area alumina (mainly boehmite). Adsorption of water or hydrocarbon is discussed in terms of various possible intermediate species and adsorption sites. IR spectra on CO adsorption were used to identify the effect of temperature on the evolution of Lewis acid sites on the two alumina surfaces. Electrical properties were dominated by protonic conduction, with H3O+ and C3H7+ as possible mobile species at low temperature. The increase of electrical conductivity (above 300 degreesC for gamma-Al2O3 and at 200 degreesC for the other sample), slightly higher in presence of propylene, was in good correlation with the catalytic behavior in propylene oxidation. This increase regained also in tan delta-temperature plots was assigned to the lattice mobilization. (C) 2003 Elsevier Science B.V. All rights reserved.