Viscosity is generally cited as a chief cause for any decrease in efficiency noticed in concentration processes, particularly with undeslimed feeds, despite the fact that there is little conclusive information available on the importance of viscosity. The literature on viscosity as applied to gravity processes is critically reviewed. The dynamic flow characteristics of natural slimes containing a large amount of ultra-fine particles-exclusively clay is studied at different slime concentrations and pH. Except for some tendency to gel at low pH with low rates of shear, they behave almost as Newtonian fluids with extremely low apparant viscosities even with about 55 %-5 mu m clayey matter. At intermediate concentrations suspensions show strong gelling behavior and pH effects become pronounced. At rates of shear on concentrating equipment, pH influence becomes less significant and apparent viscosity drops greatly. A mathematical analysis of dynamic flow characteristics of thin films is made as a function of distance from the surface of the deck. The viscosity effect due to ultra-fine particles is negligible under conditions prevailing in slime gravity processes.