Sile Region, northeastern Istanbul, includes the most important underclay deposit of Turkey In the area, Upper Cretaceous calc-alkaline volcanic rocks are the major parent rocks of the Neogene sequence composed of plastic clays, underclays, sands, gravels and Quaternary alluvial deposits. The Sile Region consists of discontinuous cyclic thin coal beds and industrial clay deposits accumulated in some lacustrine basins. Several coal seams occur within the Upper Miocene succession with gray to greenish underclays; they occured in swamp environments, coal-forming dysaerobic environment, due to fluctuation of lake water level that provided the necessary humic and fulvic acids for the post-deposition chemical leaching of highly altered andesitic rocks to kaolin in dysaerobic, relatively low pH, conditions in saturated groundwater zones. These clays are composed of disordered kaolinite, illite, mixed-layer illite-smectite, smectite, gibbsite, quartz, pyrite, anatase, K-feldspar, goethite and organic matter. The underclays have been studied because this area is the biggest sedimentary industrial clay deposit of Turkey and economic importance. Mineralogical, chemical and technological tests have been conducted to assess their potentials in the ceramics industry for the production of tiles. They were tested at so C intervals from 1000 to 1200 C. The technological properties of underclays are characterized by low values of shrinkage and flexural strength after pressing, drying and firing. Water absorption values decreased below 5 wt. % at 1200 degrees C and these underclays can be used in the manufacture of vitrified tiles or sanitary ware. The chemical analyses generally show average silica, high potassium and medium alumina contents in all samples, typically of kaolinized and illitized materials. The Sile underclays exhibit very low S and Fe contents, but low plasticity and excessive refractory properties. Firing shrinkage increases slightly with increasing temperature, but there is a systematic increase in flexural strength.