A study is reported of the effects of changing the binder viscosity in rotating drum granulation of a narrow size fraction of an irregularly shaped sand. Silicone fluids, having viscosities in the range 20-500 mPa s, were used as binders. The size distribution of granules was determined by analysis of microscope images and the granule morphology by examination of sections of granules. The compressive strength of granules was also measured. It was found that the viscosity of the binder affected both the rate of size enlargement and the mechanism of size enlargement. The growth rate increased with increase in binder viscosity up to maximum at a viscosity of about 100 mPa s. Enlargement occurred by a layering mechanism. With binders of viscosity greater than 100 mPa s, layering was not observed and growth was found to be by coalescence. Stokes number analyses of the internal deformation on impact and of the adhesion on impact of surface-wet granules were made and found to account, in part, for the effects of changing binder viscosity.