Glass ballotini of surface mean size 10, 19 and 37 mu m were granulated in a 30 cm diameter tumbling drum. Water, glycerol and surfactant solutions used to vary the viscosity and surface tension of the binder liquid. The change of porosity with time was found to be well described by a simple, empirical, exponential decay equation : epsilon-epsilon(min)/epsilon(0)-epsilon(min) = exp(-kN) where epsilon is the average granule porosity after N drum revolutions, epsilon(0) is the initial average porosity of the feed, epsilon(min) is the minimum porosity reached by the tumbling granules and k is the consolidation rate constant. epsilon(min) and k were complex functions of binder and solid properties-decreasing particle size and increasing liquid viscosity both reduced k; decreasing surface tension increased epsilon(min); and the effect of binder content on epsilon(min) varied with binder viscosity. Where possible, these relationships were expressed quantitatively and compared with consolidation models in the literature. k was also related to the previously measured dynamic yield stress of the granules. The results show that capillary, viscous and friction forces all play a role in granule consolidation.