The colloid stability of gibbsite dispersions has been investigated at high pH, near and under conditions which reflect the Bayer process for the production of alumina. These dispersions display unusual colloid stability at high pH and electrolyte concentration, as shown by coagulation rate and rheological and interparticle force measurements. There is a repulsive, non-DLVO contribution to the yield value at pH values between 10 and 12. At high concentrations of NaOH and dissolved Al (cf. Bayer conditions), strong repulsive forces are evident. For particles of colloidal dimensions, the forces are of two types, long-range hydrodynamic and shorter range steric or structural forces. A steric layer thickness of about 20 nm at the gibbsite-solution interface is indicated.