The interaction forces between gibbsite particles aging in synthetic, supersaturated sodium and potassium aluminate solutions (Bayer liquors) have been investigated. Compliance with DLVO-type behavior was observed in dilute electrolyte. Under synthetic Bayer conditions, non-DLVO, repulsive steric forces exist. Upon aging, this steric repulsion gradually diminishes and disappears. A structured interfacial layer, displaying inner and outer regions of different compressibility, develops as aging occurs and interparticle adhesion increases. At high particle approach velocities, hydrodynamic forces exist which are dependent upon the viscosity of solution. The steric, adhesive, and hydrodynamic interactions are all greater in NaAl(OH)(4) compared with KAl(OH)(4) liquors of the same concentrations and degree of aging. Solution AI(OH)(4)(-) ion-mediated interfacial structuring is apparently responsible for the time-dependent repulsive and adhesive forces. These interfacial phenomena are perceived to be precursors to, or part of, gibbsite crystal growth and agglomeration mechanisms.