The long-range order of pillared interlayered clays (PILCs) after acid activation with 0.05N HCl has been investigated by X-ray diffraction (XRD) methods. The data show that long-range order in PILCs decreases as AlCH-PB = ZrCH-PB » Zr/AlCH-PB - Cr/AlCH-PB (PB - pillared bentonite; MCH = metal chlorohydroxy pillaring agent, where M - Al, Zr or Cr). Apparently, pure oxide clusters are more stable than mixed oxide clusters. Treatment of PILCs with dilute HCI at 25°C is less damaging than at reflux temperature, and calcined PILCs are more stable than air-dried materials. More structural damage occurs with 3M sulfuric acid treatment than with dilute HCI. Treatment with a weak base also causes some degradation of the pillars. 27AI-MAS NMR has been used to study pillared hectorite (PH), as well as other clay systems. The large increase of the observable octahedral aluminum (AI(VI)) resonance seen after pillaring is explained by loss of water from the [Al13O4 (OH) 24 (H2O)12] 7 +(Al13 ) cation. 27A1 spectra of PILCs derived from different pillaring agents and exposed to various heat and acid treatments are remarkably similar.