A high water holding capacity of oil sands fine tailing has been attributed to the presence of ultrafine (<0.2 μm) clay fractions. On the basis of hydrophobic character two major types of ultrafines are recognized: biwetted, associated with a significant coverage or organic matter and preferentially hydrophilic solids. The effect of biwetted solids on the colloidal stability of ultrafine clays in aqueous suspensions has been studied by dynamic light scattering and 2H n.m.r. methods. The organic matter associates with the surfaces of the biwetted solids is believed to be responsible for their accelerating effect on aggregation. The results indicate that prevention, or reduction, of the amount of biwetted solids entering the tailings pond could be beneficial.