Two types of polyoxoaluminum solutions, base-hydrolyzed A1Cl 3 and aluminum chlorohydrate (ACH) with OH/A1 ratios of 2.42 and 2.50, respectively, have been used as reagents for the pillaring of the smectite clays montmorillonite and nontronite. Selective adsorption studies demonstrate that the method used to dry the flocculated clay layers is far more important than the choice of pillaring reagent or clay layer charge in determining the apparent pore size of pillared products. Air-drying leads to zeolite-like products with a pore opening ⪢6.2 Å and <9.2 Å, whereas freeze-drying affords pillared products capable of adsorbing molecules with kinetic diameters ⪢10 Å. The differences in apparent pore openings and the dependence on the drying method is explained in terms of a clay flocculation model in which both lamellar (face-to-face) and delaminated (edge-face, edge-edge) association of the layers can occur, depending in part on the morphology (aspect ratio) of the layers. Evidence for the presence of delaminated aggregates is provided by the results for laponite, a synthetic hectorite with a relatively small layer aspect ratio and a high tendency to form delaminated aggregates even under air-drying conditions. Layer delamination can also have important catalytic consequences, as demonstrated by differences in the sensitivity of air-dried and freeze-dried pillared clays to coke formation in the dealkylation of β-isopropylnaphthalene. Although 27Al NMR spectroscopy indicates the polyoxo cation nuclearity to be larger in ACH than in base-hydrolyzed AlCl 3, both reagents give pillared products with similar aluminum contents, pore sizes, thermal stabilities and catalytic properties. These similarities suggest the same type of oxo cations, probably Al 3 Keggin ions, are formed on the intracrystal surfaces. Also, the amount of Al bound per unit cell of pillared clay varies only over a small range (2.78 – 3.07), and the variation is not correlated with layer charge. This latter result suggests that a more or less uniform monolayer of hydrated polyoxo cations is formed in the interlayers, and that electrical neutrality is achieved through hydrolysis of the pillaring cations.