Quantitative 27 Al nuclear magnetic resonance (NMR) spectroscopy was used to examine the partial hydrolysis with NaOH (OH/A1= 1.5, 2.0, 2.25, 2.5), and subsequent ageing at 353 K, of aqueous AlCl 3 (0.25 mol dm −3 ). The sharp resonance of the tetrahedral aluminum (Td-Al) at the centre of the tridecamer [AlO 4 Al 12 (OH) 24 (OH 2 ) 12 ] 7+ , or Al 13 , was monitored to evidence the formation and decomposition of this cluster cation (a commonly used intercalant for interlayered clay minerals). The maximum amount of Al 13 forms just after a turbid solution becomes clear (ca. 4 h at 353 K). After two days ageing, Al 13 is largely transformed into A1 24 O 72 (a ‘defective’ Al 13 dimer). This preparation and ageing method is reproducible. An indirect method for verifying Td-Al substitution, knowing Al 13 decomposition kinetics, is proposed.