The synthesis method of a stable, large-pore tantalum-pillared montmorillonite family (Ta-PILC) and its physicochemical characterization are reported. The clay used is a sodium montmorillonite. The tantalum pillaring solution is prepared by careful control of the hydrolysis of a tantalum alkoxide precursor, Ta(OC2H5)(5), in an ethanolic acidic solution. From XRD analysis it has been concluded that the Ta-PILC possesses a high thermal stability, the pillared structure is preserved even after calcination at 600 degrees C, and the basal spacing, 26 Angstrom at 500 degrees C, is one of the greatest values described in the literature. On the basis of the bibliographic data concerning tantalum alkoxide hydrolysis and condensation, and taking into account the experimental results obtained for this series of Ta-PILCs, a structure of type [Ta8O10(OR)(20)], R = H, C2H5, has been proposed as the precursor molecule of the tantalum pillars.