The effect of air humidity on flow properties of two different cuts of fine glass beads was experimentally studied by means of shear tests. The moisture content of powder samples was conditioned by humid air at relative humidities between 13% and 98% in a fluidization column. In spite of the very low moisture contents in the powder (<0.2%) obtained by this technique, a significant change in the powder cohesion was observed, while the effect on the angle of internal friction was limited. A model based on the Kelvin equation and the Laplace Young equation was applied to describe the capillary condensation between touching asperities found on the surface of neighbouring particles and to estimate the relevant interparticle forces. This result was used to derive the powder tensile strength following the Rumpf approach and compare it with values derived from shear experiments. A single value of the unknown capillary bridge gap was chosen on the basis of the observed dependence of interparticle cohesive forces. This gap was able to provide quantitative agreement between model predictions and experimental results at relative humidity lower than 80%. Comparison between the total moisture content and the amount of water in liquid bridges indicated that water mainly condenses on rough surfaces of the particles and only a small portion of this condensed humidity contributes to change the powder flow properties by interparticle capillary forces. (C) 2010 Elsevier B.V. All rights reserved.