Molecular dynamics simulations of the surface of gamma-Al2O3 have been carried out at 1000 K with three different La coverages in order to study the thermostabilising effect of rare earth cations. The system consisted originally of a gamma-Al2O3 sample constituted of 1440 particles from which 10, 15 and 20 aluminium cations of high coordination were replaced by lanthanum cations. The thermostabilising effect is described in terms of the hindering of migration of tetrahedral aluminium cations by the presence of highly coordinated lanthanum cations.