The structure and energetics of bayerite and gibbsite have been investigated at the periodic ab-initio quantum-mechanical level by using a Gaussian type basis set and the B3LYP Hamiltonian. Both systems have layered structure, with intra-layer and inter-layer hydrogen bonds (HBs). In gibbsite the latter are stronger than in bayerite: 23.15 and 17.59 kJ/mol, respectively. The formation Gibbs free energy has been calculated, including entropic and enthalpic contributions: at 298 K gibbsite is more stable than bayerite by 7.74 kJ/mol. (C) 2008 Elsevier B.V. All rights reserved.