In this study, we used atomic force microscopy (AFM) to measure and identify the nature of the forces acting between single pharmaceutical powder particles, namely a crystalline and a semi-crystalline form of lactose. The experiments done at different moisture level, namely from 20% to 60% of relative humidity, identified capillarity as the main source of adhesion between single particles. A strong correlation was observed between the AFM adhesion results and cohesion measurements taken with a FT4 torsional type powder shear tester. This correlation between adhesion and cohesion allows the identification of capillary forces as the main source of the cohesion measured in shearing experiments in ambient moisture conditions. In the case of the semicrystalline lactose powder, a sharp decrease in both the adhesion and cohesion at 50% RH was linked to absorption of water accumulated at the particle surfaces to fuel a phase transition of the non-crystalline domains to crystalline lactose, hence limiting the capillary action between particles at this specific moisture condition. (C) 2015 Elsevier B.V. All rights reserved.