Characterization of the aluminum-containing species in freshly prepared, supersaturated caustic aluminate solutions has been carried out by performing static light scattering on three series of solutions: constant NaOH concentration, 4.00 and 6.00 M, with varying Al(OH)(3) concentrations, 0.39-4.68 M; and constant molar ratio OH/Al = 4.37, with NaOH concentrations of 0.10-6.00 M (Al(OH)(3) concentrations of 0.07-4.38 M). For the two sets of solutions containing constant NaOH concentrations, the intensity of the light scattered did not change over the concentration range of Al investigated. This implies that the solution speciation is independent of the Al concentration and therefore, for these solutions, a conventional Hc/tau versus c analysis to determine the weight average molecular weight or the second virial coefficient of the Al-containing species present is not valid. Analysis of the constant OH/Al ratio solutions revealed markedly different behavior for the dilute extreme of the concentration range. Up to an Al(OH)(3) concentration of 0.81 M (Al of 0.022 g cm(-3)) increases in the intensity of light scattered were observed. Above this concentration the intensity of light scattered decreased slightly and then became insensitive to further increases in solution concentration. These results indicate that the nature of the solution species in fresh caustic aluminate liquors are insensitive to Al concentration but are sensitive to the concentration of NaOH. On increase of the concentration of NaOH in solution, the nature of the Al-containing species changes. In relatively dilute solutions the Al-containing species are relatively dense with respect to the background solvent and thus scattering light independently from the solvent. In more concentrated solutions the Al-containing species are present as large diffuse particles that are intimately a part of the solution and, hence, do not scatter light independently from the solvent. Two small- and wide-angle X-ray scattering experiments were conducted on the OH/Al = 4.37 solutions at Al(OH)(3) concentrations of 0.93 and 4.12 M (0.025 and 0.111 g cm(-3) Al, respectively). Each solution was allowed to age at room temperature, and time-resolved measurements were taken. Similar trends in the small-angle X-ray scattering (SAXS) data were observed for both solutions on aging. The SAXS data observed have been explained by the following mechanism. Aluminum hydroxide "nuclei" of approximate diameter 7 Angstrom form in solution. The initial growth path for these nuclei is via addition of small solution species, and hence a continuous particle size distribution is formed. Upon aging, the concentration of the nuclei in solution decreases and their growth path becomes dominated by agglomeration. This process occurs more rapidly in the dilute solution as compared to the concentrated solution and may reflect the difference in the nature of the particle interfacial region as implied by static light scattering; i.e., the less diffuse particles agglomerate more rapidly and hence particle growth in dilute solutions is more rapid. Additionally the WAXS data show the presence of gibbsite diffraction peaks in the dilute solution after 17 h of aging, whereas these are not present in the concentrated solution even after 45 h.