Bentonite deposits in Basoren Kutahya, West Anatolia, Turkey formed from alteration of perlite and pyroclastic rocks of Pliocene age. The distribution of bentonite deposits along faults in the study area indicates that the alteration solutions were hydrothermal. Although lateral zonation is observed in bentonite deposits in some regions (i.e.Demirli, Akyokus, SekliceSariokuz, etc.), alteration zones are extremely irregular in the Cayirlik bentonite deposit. X-ray diffraction studies have shown that Basoren bentonites contain dioctahedral Ca-smectite. The Greene-Kelly test (Li-saturation and heating) showed that the Demirli and Akyokus bentonites consist of montmorillonite and that the Cayirlik bentonite consists of montmorillonite and/or beidellite. Spherulitic or hemispherical 'crystals' of opal-CT minerals formed from hydrothermal alteration of volcanic glass. The MgO, CaO and total Fe2 O3 enrichment in bentonites, compared to parent rocks, is related to the chemical composition of hydrothermal solutions that passed through the ophiolitic rocks such as serpentinite.