We report on a study of adsorption of probe molecules on the aluminum sites of gamma-alumina surface planes. DFT electronic structure and total energy calculations for (100) and (1100) surface planes were carried out in order to find the reactive sites. The adsorption of two molecular species (water and hydrogen sulfide) was studied using a DFT plane-wave pseudopotential method. We found that both molecules were adsorbed on the (100) surface and that both molecules were dissociated on the (1100) surface. Consequently, the (110D) surface is more reactive than the (100) surface.