Solutions containing hydroxy-SiAl(HSA) oligocations were prepared by two procedures: (1) treatment of a mixture of silicic acid, which was prepared by dispersing silica sol in an ultrasonic generator, and A1C13 with aqueous NaOH, followed by aging of the product and (2) preliminary preparation and aging of hydroxy-Al13 oligocations followed by reaction of the latter with silicic acid. Exchange of Na-montmorillonite with HSA oligocations yielded pillared clays (designated as SiAl-PILC obtained using method 1, and GSiAl-PILC obtained using method 2) showing mixed maximum d(001) values of 2.65 and 1.90 nm for air-dried samples and maximum surface areas of ~267 m2/g after outgassing at 200 °C/101 kPa. Elemental analysis showed a gradual increase in the Si/Al ratio in the intercalated HSA oligocations with increasing Si/Al ratio in the pillaring solution. The pillaring structure, thermal stability, and surface acidity of the pillared clay products have been determined using 27A1-NMR, X-ray diffraction (XRD), XPS, DTA, IR, and temperature-programmed desorption (TPD) measurements. The results suggest that the thermal stability of the pillared clays was improved by incorporating silica. Hydroxy-SiAl pillared clays show sharply higher concentrations of strong acid sites, especially Bronsted acid sites, compared with reference Al-PILC obtained by pillaring with hydroxy-Al13 oligocations. This increased acidity is probably due to the presence of acidic, surface silanol groups in the HSA oligocations. The cracking of cumene and the disproportionation and alkylation of 1,2,4-trimethylbenzene were carried out in a pulse reaction system at 400 °C over the pillared clay catalysts. Incorporating silica into an alumina pillar increased the cracking activity of cumene and the activity and selectivity for the disproportionation of 1,2,4-trimethylbenzene compared to those observed for Al-PILC.