The adsorption of benzidine by sepiolite obtained from Eskisehir (Turkey) has been studied by Fourier-transform infrared spectroscopy. A deep green colouration results on reaction of benzidine and sepiolite. In the IR spectrum of benzidine treated sepiolite ν(C-N+) vibrational modes of the uni- and di-positive radical cations are observed in addition to vibrational modes of coordinated benzidine. Spectroscopic results indicate that the principle adsorption mechanism for benzidine is coordination to surface hydroxyls from nitrogen lone pairs and to a relatively small extent, formation of radical cations. No physical sorbed species have been detected. It must be noted that the adsorption of benzidine affected the hydroxyl group vibrations of sepiolite.