The mechanism of Cu(II) sorption on montmorillonite was studied with electron paramagnetic resonance (EPR) spectroscopy. The major sorbed species were successfully identified by EPR spectroscopy as they showed distinct signals due to their strength of binding and local structure. The EPR results together with macroscopic sorption data show that the sorption involves at least three different mechanisms. The dominant sorption mechanism changed with pH and Na+ content in solution. In the acidic pH range, the sorption is independent of pH but is dependent on Na+ concentration. Like free copper ions, the sorbed Cu(II) shows an isotropic absorption line. It is thought that the Cu(II) is ion-exchanged in the interlayer site and is capable of free tumbling motion despite electrostatic binding force. In the near neutral pH range, the sorption is strongly pH dependent and the sorbed Cu(II) shows no EPR signal. It is interpreted that the Cu(II) is sorbed by a surface complexation mechanism to form an inner-sphere surface complex. The EPR spectrum with a dipolar splitting pattern means that the dimeric Cu(II) surface species is the dominant sorbed species in the basic pH region where the Cu-Cu internuclear distance of the dimer is estimated to be similar to 3.3 Angstrom.