The effect of tetrabutylammonium (Bu4N+) and tetrapentylammonium (Pe(4)N(+)) cations on the modification of the organo-montmorillonite structure upon acid-treatment was investigated. Samples were treated with HCl for various times (2-12 h). Structural changes were followed by MAS NMR spectroscopy. The Si-29 MAS NMR spectra of initial Na-saturated form (Na-SAz) showed gradual decrease of the intensity of the resonance assigned to SiO4 cross-linked in the tetrahedral sheets Q(3)(OAl) while signals arising from the reaction product Q(3)1OH and Q(4)(OAl) became more pronounced upon acid treatment. The Q(3)(OAl) signal almost completely disappeared for Na-SAz treated for 8 h on contrary to Bu4N-SAz and Pe(4)N-SAz showing signal of relatively high intensity even after 12 h. The Al-27 MAS NMR measurement proved that more than one half of Al remained in the reaction product of Bu4N-SAz and Pe(4)N-SAz after 8 h treatment, while Al content dropped below 5% for Na-SAz. Formation of acid sites was investigated via pyridine adsorption. Only physically adsorbed and H-bonded pyridine was detected for acid-untreated samples. In contrast, the IR spectra of the samples partially decomposed in HCl revealed bands of pyridine adsorbed on Bronsted acid sites. Strongly bonded pyridine was able to bear up heating even at 230 degrees C. (C) 2012 Elsevier Inc. All rights reserved.