The increasing use of nanoparticles in the production of nanomaterials has led to the identification of nanoparticle agglomeration in air or in a given medium as a key problem, both on the standpoint of process performance (i.e. product homogeneity) and above all on inhalation risks. This paper suggests a new method to evaluate the structure and the dynamics of agglomeration of a nanopowder, based upon monitoring of the shear stress of a powder which is submitted to the mechanical solicitation of a rheometer. At low and increasing shear rates, the powder flow will evolve from a Newtonian regime (dense powder) to a Coulombic regime (slightly rheofluidized dense phase). At larger shear rates, the powder will be set in suspension, which characterizes a kinetic regime governed by particle collisions. The specific shear energy, related to the specific agglomeration energy, has been calculated for different nanopowders (carbon, aluminum, silica) and compared to non-cohesive micrometric powders (glass beads). From the measurements of shear rates and stresses at the frictional/kinetic transition, agglomerate diameters have been evaluated for carbonaceous material and for silica. Values of these agglomerates can range from 200 to 500 pm and are related to their propensity to break apart. The carbonaceous materials seem to be the more difficult to deagglomerate, whereas silica nanopowder agglomerates are more easily breakable. Measures of cohesiveness (or specific agglomeration energy) can be useful for assessing the dispersibility of nanopowders and their relationships to inhalation or explosion risks. (C) 2013 Published by Elsevier B.V.