The interaction of water or propylene dipoles with surface of gamma-Al2O3 and of a boehmite type alumina between room temperature and 400 degreesC was evaluated from evolution of the apparent capacity/dielectric constant during thermal cycling and under flushing with dry or humid gases (argon, oxygen) and with a mixture containing propylene and air (i.e. in conditions which V, mimic the catalytic experiment). Data allowed to stipulate that the extent of weak adsorption was higher on gamma-Al2O3, in relation with the higher amount of surface OH groups, as supported by IR data. (C) 2003 Elsevier Science B.V. All rights reserved.