Links between flow properties and formulation of powders of 100 and 500 mu m mean particle sizes were investigated. To determine the influence of surface treatment, the flow properties of glass beads were analyzed after various surface treatments leading to hydrophilic, hydrophobic, and lactose-coated surfaces. Furthermore, to investigate the influence of powder core composition, agglomerated lactose powders of circa 100 and 500 mu m mean particle size were also produced by high-shear wet granulation and characterized. Hydrophilic and hydrophobic surface treatments did not alter surface topography and particle size distribution, whereas lactose-coated glass beads and agglomerated lactose powders presented noticeable changes of surface structure and particle size increase. Furthermore, all 100 mu m powders were classified as easy flowing; hydrophobic glass beads and agglomerated lactose presented the highest and lowest powder flowability, respectively. For 500 mu m powders, hydrophilic glass beads and agglomerated lactose powders had the highest and lowest flowability, respectively. The poorer flowability of agglomerated lactose may arise from their angular shape, their higher width of particle size distribution, the lower core density and the higher cohesion of lactose-coated particles. Last, no significant difference of powder compressibility was observed and all studied powders were hardly fluidizable, due to their high particle weight.