This article describes the work which defined the relationship between the viscosity of nanofluids, containing silica nanoparticles, temperature and the concentration and the size of the particles. The nano-powders produced by two different methods have been used and the results compared. The difference in a way they affect the viscosity of the nanofluids has been studied and explained. It was identified that the viscosity depends on the value of a specific surface in accordance with the linear law. The mechanism to increase the value of the volume ratio of the solid phase by the means of associated layer of the colloidal particles has been proposed. The assessment has been made of the thickness of the associated layer of liquid on the surface of the silica nanoparticles which have been dispersed in water and epoxy resins. The application of the Bachelor formula has been demonstrated for the nanoparticles taking into account the volume ratio of the associated layer of liquid. (c) 2014 Elsevier Ltd. All rights reserved.