Widespread alteration in the Miocene lacustrine volcanic/sedimentary rocks of the Ankara-Cankiri basin of central Anatolia has resulted in the formation of sizeable (economic) quantities of bentonite deposits. No detailed characterization of the geological, mineralogical, and geochemical properties or the depositional environments of these primary and secondary bentonite deposits has been carried out to date. The present study was undertaken to close this knowledge gap. Two possible hypothetical processes were examined to explain the genesis of the bentonites: 1) The bentonites were formed by the devitrification of volcanic glass in a lacustrine setting; and 2) The bentonites were formed by the chemical weathering of previously deposited volcaniclastic sediments and ophiolitic materials. The characteristics of the bentonites were examined using X-ray diffractometry, scanning and transmission electron microscopy, energy dispersive spectroscopy, and chemical analyses of major and trace elements. The Ankara-Cankiri bentonites are found gradationally interbedded with parent Miocene volcanic and volcaniclastic rocks. These bentonites were deposited in a shallow lacustrine setting based on observed desiccation cracks, locally enclosed coal seams, plant rootlets, gypsum lenses, yellow sulfate-like fracture infillings, and ferric iron oxide stains. Smectite resulted from the chemical weathering of feldspar and possibly also the weathering of biotite and hornblende. This smectite was precipitated in situ on partially dissolved glass and feldspar. The average major-element composition of the smectite-rich clay fractions yielded the following montmorillonitic smectite structural formula: (Na0.33Ca0.31K0.18) (Al2.35Fe0.80Mg0.78)(Si7.79Al0.21)O-20(OH)(4).