The distribution of 14 lanthanide elements (Ln) between water and montmorillonite's surface was simultaneously determined by ICP-MS. From the dependence of the distribution coefficients on the atomic number of Ln, the formation of outer- or inner- sphere complexes with the surface of montmorillonite was estimated. The formation of the outer-sphere complex was also confirmed by the quenching of the fluorescence from Eu(III) ion due to the hydrated water.