The Sogut kaolinite deposits at Calti, Inhisar, and Kure, Turkey, are an important source of raw materials for the ceramics industry of that country, but no detailed mineralogical or geochemical characterizations of these economically important materials have been carried out to date. The purpose of this study was to fill this gap by performing mineralogical, geochemical, and isotopic characterizations of these kaolinite deposits which occur within Paleozoic granite-granodiorite complexes that are crosscut by aplite and pegmatite dikes, and overlain by Neogene sedimentary units. These units are dominated by quartz veins and networks of subvertical fractures and weak zones that were invaded by hydrothermal fluids, resulting in their kaolinization and silicification. Altered units and related host rocks were examined using polarized-light microscopy, X-ray diffractometry, scanning electron microscopy, infrared spectroscopy, and chemical and isotopic methods. Feldspar crystals are either serieitized or kaolinized, and mica exhibits partial chloritization; Fe-Ti-Mn oxides occur within fractures. Kaolinite crystals occur in authigenie vermiform or plate-like stacked forms, having contacts with resorbed feldspar crystals which locally exhibit thick, platy, and subparallel orientations relative to microfractures, the pathways for hydrothermal-fluid injection. Altered feldspar reliets are associated mainly with kaolinite, smectite, quartz crystals, and illite/mica. Increase in (Al+Fe)/Si in the kaolinized units (relative to host-rock granite and granodiorite complexes and silicification), depletion of Ba+Rb, and a negative Eu anomaly reveal that the alteration of feldspar by hydrothermal fluid, the character of which was determined from O- and H-isotopic values, resulted in the precipitation of kaolinite. Thus, the Sogut kaolinite deposit possibly formed by hydrothermal alteration and a feldspar dissolution-precipitation mechanism in both the granite-granodiorites complexes and related overlying sedimentary units under acidic environmental conditions, which developed via depletion of the soluble elements Na and Ca.