Surface force, rheological and crystallization techniques have been used to investigate interparticle forces and their link to interfacial structuring and agglomeration of gibbsite crystals in pure, supersaturated sodium and potassium aluminate solutions (synthetic Bayer liquors). Initially repulsive interparticle forces exist; these attenuate with time, and are followed by the emergence and growth of adhesive forces upon further aging. The adhesive forces were smaller at the gibbsite (001) crystal face compared with other faces and their rate of growth greater in sodium aluminate than in potassium aluminate solutions over similar aging periods. The rheological data correlated well with the interparticle force determinations. Both the thixotropic structure and yield stresses increased with time and were greater in sodium-based than in potassium-based liquors. SEM and crystal size analyses of the crystallized product showed that agglomeration was more pronounced in sodium than in potassium-based Bayer liquors and was more prevalent between non-(001) crystal faces. For the size enlargement of sub-micron sized gibbsite crystals, agglomeration, rather than crystal growth, was the predominant mechanism and is directly controlled by particle interaction farces.