The Hg L-3-edge XANES spectra of montmorillonite treated with Hg(NO3)(2) 0.01M aqueous solution and the mixing solution of Hg(NO3)(2) 0.01M aqueous solution and cysteine 0.01M aqueous solution have been measured at several high temperatures. The adsorption and desorption of mercury in Hg-rich montmorillonite minerals and the effect of cysteine on the adsorption of mercury by this clay mineral were studied. Our analysis shows that mercury is six fold coordinated by water molecules and the oxygen atoms of montmorillonite surface without the amino acid introduction. Mercury atoms prefer to bond with sulphur atoms of the amino acid cysteine to form more stable Hg-S bonds when the amino acid cysteine has been introduced. This result suggests that the treatment with the amino acid enhances the stability of mercury sorbed by the montmorillonite mineral and efficiently retard the release of mercury ions into the geosphere.