Montmorillonite and attapulgite, in concentrations of 3% and above, are very effective in the retardation of settling of ThO/sub 2/ slurries and in the prevention of hardcaked sediments. The most useful material found was attapulgite sold under the trade name of Zeogel. Concentrations of 3% were very effective and 5% were even more so. Viscosity of the slurry may be prohibitive in concentrations above 3%. The most desirable pH range appears to be from 7.5 to 9.0 both from the standpoint of rate of settling and hardness of sediment formed. In the preparation of slurries containing these materials, it is essential that the suspending agent be mixed in the water using a high shear type mixer. (auth)