Montmorillonites and saponites have been pillared with oxohydroxy-aluminium solutions (OH/Al molar ratio of 1.6). The solids have been examined by X-ray diffraction. The textural properties, residual cation exchange capacities, and acid contents have been established. The pillared clays exhibit thermally stable spacings, specific surface areas, and micropore volumes typical of such materials. Higher acid contents were found for pillared saponites than for pillared montmorillonites. IR spectroscopy of pillared saponites showed a new OH stretching vibration at 3595 cm(-1), absent in pillared montmorillonites. The acid character of these OH groups has been assessed by pyridine adsorption. The catalytic properties have been evaluated over Pt-impregnated samples in the heptane hydroisomerisation reaction. Al-pillared saponites, smectites with lattice Si for Al tetrahedral substitutions, showed superior catalytic performances (high conversions, high yields of isomers) compared with Al-pillared montmorillonites, clays with lattice octahedral substitutions. The higher activity of the saponites is attributed to the higher content and strength of the acid sites associated with Si-OH...Al groups produced upon proton attack of the tetrahedral Si-O-Al bonds. Such strong acid sites are absent in Al-pillared montmorillonites.