The total surface free energy of nanometric particles in contact with a liquid electrolyte can be easily accessed through potentiometric measurements. This requires a special though simple designed set-up, and a careful chemical and textural characterization of the powders. This is here first exemplified for non-stoichiometric NiO, in contact with Ni2+ aqueous solutions, which total surface free energy was evaluated to 0.34-0.36 J/m(2) for monolithic particles, independently of the level of non-stoichiometry. Much larger values are systematically observed for mosaic powders; hence indicating a large contribution of the solid-solid inter-crystallites interfacial energy or strains. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.011201esl] All rights reserved.