The sol-gel transformation of aqueous solutions of aluminium ions into aluminium (oxy)hydroxides induced by the addition of a 'soft base'-'Tris-buffer' (pK(a) = 8.2) has been investigated using monotonous single-batch titrations and a combination of four complimentary techniques for monitoring pH, conductivity, viscosity and ultrasound parameters (velocity and attenuation). The multi-probe monitoring of the formation of aluminium (oxy)hydroxides enabled important stages of the sol-gel transformation process including: the structural conversion of aluminium Keggin-like polynuclear clusters into nanoparticles of aluminium (oxy)hydroxide; the aggregation of primary nuclei of aluminium (oxy)hydroxide into larger particles, and the 'arrested growth' of the aggregates with the formation of the three-dimensional gel network to be followed. The effect of aluminium ion molecular speciation on the sol-gel transformation stages is discussed. The data presented show that ultrasonic spectrometry, although a novel tool for sol-gel studies, is beneficial for the non-invasive monitoring of the latter stages of aluminium (oxy)hydroxide formation and its eventual breakdown in the presence of excess base. (c) 2008 Elsevier Inc. All rights reserved.