Highly dispersed Pt particles on gamma-Al2O3 are known to catalyze dehydrogenation and hydrogenolysis of alkanes under H-2 pressure which are of main concerns within the context of reforming catalysts. We first highlight recent progresses on the density functional theory (DFT) simulations of gamma-Al2O3 surfaces, a prerequisite step, and then address the challenging question of gamma-Al2O3-supported Pt clusters in reactive conditions. For that purpose, we report a DFT study of the thermodynamic stability of CxHy (with x = 1, 2 and 0 <= y <= 5) species formed upon dissociative adsorption of ethane on Pt-13/gamma-Al2O3 models at variable H coverages. The impact of the pressure ratio J = P(H-2)/P(C2H6) is thus quantified and rationalized as a competition between CxHy and H species for cluster sites, combined with cluster morphology effects (reconstruction at high H coverage). We finally discuss these results in detail with the experimental literature. (C) 2013 Elsevier Inc. All rights reserved.