Changes of electrical conductance and capacity of gamma -Al2O3 were studied in situ during dehydration/rehydration in gas flow. The measurements were performed at atmospheric pressure by following a standard protocol of experiments, and were coupled with simultaneous monitoring of the composition of the inlet/effluent gas. The electrical properties are dominated by the protonic conduction. It was shown that at low temperature this occurs by vehicle mechanism, being facilitated by the presence of water molecule adsorbed on gamma -Al2O3 surface; the decrease of electrical conductance on heating is obviously connected with dehydration. At higher temperature (t > 200 degreesC) conduction is dominated by proton hopping between OH groups (Grotthuss mechanism). It is suggested that in current operating conditions in catalysis, gamma -Al2O3 is only partially dehydrated/dehydroxylated, the bulk acting as a source of water/OH groups for the surface; this process controls the acidity of alumina and alumina-supported catalysts.