Abstract Polynuclear hydroxyaluminum-montmorillonite complexe were prepared by treating Na-saturated montmorillonite with partially neutralized solutions of Al-containing monomer and polynuclear hydroxyaluminum species. The polynuclear hydroxyaluminum cations were preferably adsorbed by montmorillonite over monomer Al. The maximum quality of the polynuclear hydroxyaluminum adsorbed was ≈400 meq Al/100 g of montmorillonite and was independent of OH/Al molar ratios of hydroxyaluminum solutions. Three types of complexes were characterized by X-ray diffraction analysis; 18.8 Å phase, 28 Å phase and an intermediate phase. Initial conditions for drying suspensions containing complexes were found to be determining factors for the type of solid complex formed. Extremely dry conditions favored the formation of the 18.8 Å phase, whereas ambient conditions produced the intermediate phase. On aging, these phases were gradually transformed to the 28 Å phase. Upon heating at 700°C the 18.8 Å phase was converted to the 28 Å phase. The calculated average chemical composition of the polynuclear hydroxyaluminum cations adsorbed in montmorillonite layers was [Al(OH) 0.56+ 2.44] n. The quantity of the maximum adsorption, ≈400 meq, was only a quarter of the quantity of the interlayer cations required to form a chlorite structure which has a layer thickness of 14.2 Å. In spite of low amounts of hydroxy-Al adsorbed, the observed large basal spacings indicated that the complexes found in the present investigation must have pillared structures. From X-ray data, the height of the pillars was found to be ≈9.2 Å. This and chemical data suggested that the Johansson's model molecule [Al 13O 4(OH) 24(H 2O) 12] 7+, was most suitabl e for the pillar. If this is the case, then one molecule occupied an area of 12–13 montmorillonite unit cells at every layer in the 18.8 Å phase, whereas two cations occupied the corresponding area at every second layer in the 28 Å phase. Therefore, a transformation from the 18.8 Å phase to the 28 Å phase could be explained by the migration of the cations from an interlayer space of the 18.8 Å phase to one layer above or below.