Batch adsorption experiments in the presence of oxygen were performed to study the interlayer reactions of aniline on Cu(II)-montmorillonite in aqueous solutions. At concentrations below a critical value of C-G = 2.6 mmol dm(-3) only a colored Cu(II)-aniline complex is for med, characterized by a stability constant of log(K-assoc/dm(3) mol(-1)) = 1.5. At concentrations beyond C-c aniline polymerizes yielding a dark brown product, which is identified by two vibrational spectroscopy techniques, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and FT-Raman spectroscopy. Vibrational assignment of aniline, polyaniline, and the interlayer reaction products of aniline at Cu(II)-montmorillonite was achieved by a comparison of the band position and intensity observed in the ATR and FT-Raman spectra with wavenumbers and intensities (IR and Raman) from ab initio quantum-mechanical calculations. Density functional theory (DFT, B3LYP/6-31G*) has been used to calculate the geometry, frequencies, and intensities (LR and Raman) of aniline. The geometry and vibrational calculations of a four-ring unit (emeraldine base, EB) are believed to be a good representation of the polyaniline at Cu(II)-montmorillonite. The geometry of EB was fully optimized at the Hartree-Fock level of theory. The data of polyaniline presented suggest that IR and Raman data calculated ab initio on relatively short oligomers (quantum-mechanical oligomer approach) may provide valuable information regarding the interpretation of vibrational spectra of polymers. From the comparison of experiments and calculations, it is concluded that a catalytic/intercalation polymerization of aniline to polyaniline took place inside the interlayer of the Cu(II)-montmorillonite clay mineral.