Abstract : The sorption mechanism and the crystallochemical environment of divalent Co and Zn on hectorite, a magnesian smectite, and quartz are investigated by combined kinetics chemical studies and polarized-EXAFS (P-EXAFS) spectroscopy. At high ionic strength (0.3 M NaNO3), pH 6.5, and a sorbate/hectorite ratio of -50 flmol/g, Zn . and Co adsorb via a pH-dependent mechanism. This sorption process coincides with an initiat excess release of protons and Mg in the supernatant, and with a long term inhibition of hectorite dissolution. Sorbate cations form mononuclear surface complexes in the continuity of the magnesian octahedral sheet of the clay structure. At low ionic strength, Zn and Co initially sorb rapidly by cation exchange, and form outer-sphere surface complexes. Following this fast sorption process, sorbed ions are progressively transferred from interlayer ex change sites to layer edges sites, where they form inner-sphere surface complexes. Cation exchange sites thus act as transient cationic buffer. At pH 7.3, high ionic strength and sorbate/hectorite ratio (1480 umol/g), the amplitude and kinetics of Zn sorption depend on the concentration of dissolved Si, [Si] . At [Si]= 30-60 umol/L, Zn forms small polynuclear complexes of 2-3 atoms located in structural continuity of hectorite octahedral sheets. At [Si] = 540 umol/L, heteronucleation and growth of a Zn-rich phyllosilicate occurs in structural continuity of hectorite layers. EXAFS spectra of Co-sorbed quartz first indicate formation of large Co octahedral sheets. However, .close examination of atomic contributions revealed *that these sheets are connected to Si tetrahedral sheets, meaning that the sorption of Co on quartz leads to the neoformation of phyllosilicate. The mechanism of this neoformation is discussed .