The results of EPR, UV-VIS and IR spectral studies show that benzimidazole forms various complexes with Cu(II) exchanged in the interlayer space of montmorillonite, depending on the acidity of the reaction medium during synthesis. In a nearly neutral solution a bis(benzimidazole)bis(aqua) Cu(II) cationic complex is formed. In a strongly basic solution, benzimidazole is bound to Cu(II) in the form of an anionic bifunctional ligand and a planar intercalated bis(benzimidazolate) Cu(II) complex is formed, where the benzimidazolate anion acts as a bridging ligand in the polymeric complex. The treatment in basic solution leads to partial illitization of the smectite. In a strongly acid solution, a nearly stoichiometric ionic exchange of benzimidazolium ion for Cu(II) ions takes place in the interlayer space of montmorillonite.