This work concerns the study of the experimental and theoretical behavior of cohesive powders and in particular the comparison of two categories of powders: micronic and nanometric size particles. The latter, seem to be more complex and do not present the same behavior as the micronic powders under the action of the external forces. The examples used here are for the behavior of two micronic alumina powders of 3 mu m and 13 mu m average diameter and three nanometric powders of TiO2 of 204 nm, 159 nm and 167 nm average diameter were examined. These powders belong to the group C of Geldart's classification [D. Geldart, Types of gas fluidisation, Powder Technology 7 (1973) 285-292] characterized as being cohesive powders with a non-free flow and difficult to fluidize. The physical properties of alumina and Ti02 Such as particle size, shape factor and density were analyzed rigorously to reach the measurements of the primary size and not of the agglomerate. The flow properties of the powders were characterized by measurements of the Hausner index, angle of repose, angle of slide, consolidation and shearing. The contact properties (interparticle energy) between particles were analyzed by a microscopic approach. The results suggest a more complex behavior of nanometric powders (TiO2) than that of the micronic powders. The calculation and the analysis of the particulate interactions enabled us to correlate the degree of cohesion of the powders to the intensity of the interparticle interaction. (C) 2008 Elsevier B.V. All rights reserved.