The crystallization mechanisms of colloidal Al(OH)(3) from dilute ([NaOH] <= 1.00 M, [Al(III)] <= 0.82 M), intermediate (1.00-2.50 M NaOH, 0.82-2.05 M Al(III)) and concentrated ([NaOH] > 2.50 M, [Al(III)] > 2.05 M) sodium aluminate solutions have been investigated. Bayerite (alpha-Al(OH)(3)) crystallization predominates from dilute sodium aluminate solutions, whilst for concentrated solutions the precipitated phase is predominately gibbsite (gamma-Al(OH)(3)) Solutions with an intermediate NaOH concentration gave rise to a dimorphic phase mixture. Particle size distribution (PSD) analysis of dynamic light scattering (DLS) in the particle size range of 0.05-1.0 mu m shows that in a solution of intermediate concentration (i.e. 1.50 M NaOH, 0.97 M Al(III)) progressive, uni-modal nuclei growth occurs up to an average diameter (d) of approximate to 300 nm. Thereafter, the PSD broadens with time due to aggregation until the limit of sizing by DLS (d approximate to mu m) is reached. In contrast, particle size enlargement in a concentrated solution (i.e. 4.00 M NaOH, 3.28 M Al(III)) was primarily via aggregation. Distinctly bi-and tri-modal particle size distributions are observed for d > 225 nm in the concentrated solution, indicating the existence of secondary nucleation. This is not observed in the dilute solution. The average particle size enlargement rate is nearly an order of magnitude slower in the solution of intermediate concentration comprising mixed bayerite-gibbsite precursor particles than the concentrated solution containing predominantly gibbsite nuclei. (c) 2005 Elsevier B.V. All rights reserved.