In this paper, the multimolecular adsorption equation (so-called GAB equation) was modified by adding an amount of condensed water between the particles. This new model was then compared with experimental results obtained on three nanostructured powders of different nature: zinc, amorphous carbon and zinc-aluminium obtained from metallisation fumes. The influence of the particle diameter, the porosity of the sample and the importance of the determination of the GAB constant for the new equation were all studied. We found that the new model corresponds well with experimental data on almost all the range of water activity and thus increases the range of the GAB equation. Finally, the new developed model only uses three fitted GAB parameters and does not require additional adjusting parameters. (C) 2014 Elsevier Ltd. All rights reserved.