The uptake process of Th(IV) onto montmorillonite was studied using powder and polarized-EXAFS (P-EXAFS) spectroscopy. Sorption samples were prepared in 0.1 M NaClO4 Solutions either undersaturated (pH 2 and 3, [Th](initial): 2.7 x 10(-6) to 4 x 10(-4) M) or supersatured (pH 5, [Th](initial): 4.3 x 10(-5) to 4 x 10(-4) M) with respect to amorphous ThO2. Th loading varied between 1-157 mumol/g at pH 3 and 14-166 mumol/g at pH 5 and equaled 41 mumol/g at pH 2. At pH 5 and high surface loading the EXAFS spectrum resembled that of amorphous Th(OH)4, suggesting the precipitation of a Th hydrous hydroxide. At low and intermediate surface coverage two 0 coordination shells at similar to2.24 and similar to2.48 Angstrom, and one Si shell at 3.81-3.88 Angstrom, were systematically observed regardless of pH. The formation of Th nucleation products and Th-Si solution complexes and the sorption of Th on a silica precipitate were excluded from the EXAFS spectra analysis and solution chemistry. In these conditions, Th was shown to bond the montmorillonite surface by sharing double corners with Si tetrahedra. This structural interpretation is consistent with surface coverage calculations which showed that the edge sites were saturated in the two highest concentrated samples (34 and 157 mumol/g) at pH 3.