Silica nanoparticles are often used as flow regulators applied to the surface of fine industrial powder particles, such as toners for electrophotographic (EP) printing devices. In this paper, we have investigated their effect on the cohesion and internal friction of model polymer powders (poly (styrene-co-divinylbenzene) (PS-DVB), d(50)=7.84 mu m), whose flow properties are systematically controlled by applying various different types (e.g. size and hydrophilicity) and amounts of silica nanoparticles under a normal consolidation stress of 2 kPa using a ring shear tester (RST). The ability of the nanoparticles to reduce the bulk powder cohesion has been confirmed to be dependent on how efficiently they can disrupt the direct contacts between the PS-DVB particles, resulting in a significant reduction of the interparticle adhesion. However, the cohesion increases when the agglomerates of the silica nanoparticles form a flat film structure which causes an increase of the effective contact area. On the other hand, the nanoparticles were found to work better as lubricants when the size of their agglomerates is smaller. Also, the internal friction becomes higher when the silica silica contacts become dominant, e.g. at higher actual surface coverages, but not as high as that for irregularly-shaped particles. The variation in behaviour between nanoparticles of differing hydrophilicity can be attributed to the ease of breaking down agglomerates on mixing; the cohesion of the PS-DVB powders coated with hydrophobic silica nanoparticles is smaller due to their higher actual surface coverage whilst the internal friction becomes higher. (C) 2013 Elsevier Ltd. All rights reserved.