Abstract: A study was carried out on the influence of the sequence of addition of montmorillonite, hydroxy-Al ions, and tannic acid ([Al] = 0.015 M; 6 mmole Al/g of clay; tannic acid/Al molar ratio = 0.1) on the nature of organo-clay complexes formed at pH 4.5. A negligible amount of tannate was held on the clay surfaces in the absence of Al, whereas in the presence of Al, hydroxy-Al-tannate species were easily adsorbed on clay surfaces. Their distribution on the external surface and in the interlayer space of montmorillonite, however, was a consequence of how the components reacted with each other. The complexes showed broad X-ray powder diffraction peaks at 15.6 to 19.2 Å at room temperature. They also showed different behavior to preheating, ethylene glycol solvation, and chemical treatments. Whereas tannic acid showed two prominent exothermic peaks at 390° and 510° all hydroxy-Al-tannate-montmorillonite complexes showed a broad exotherm at about 400° Some complexes showed, in addition, a small inflection between 350° and 420°C. The complexes also showed distinct differences in cation-exchange capacity, carbon content, extractable Al, titratable acidity, and mode of aggregation after drying.