gacli-Kemerburgaz area, NW of Istanbul, is one of the most important underclay deposits of Turkey This area is composed of Paleozoic sedimentary sequence, Upper Cretaceous volcanic rocks, Neogene sediments and Quaternary alluvium deposits. The calc-alkaline volcanic rocks are the parent rocks of the underclays in this area. In this work, mineralogical and chemical analyses and relevant technological tests were conducted in order to assess the potential of selected underclays of this region. The technological properties were characterized by firing shrinkage, water absorption and breaking strength after firing. The chemical analyses generally showed the characteristics of typical kaolinized materials. Underclays containing (up to 33 wt. %) porcelain stoneware formulations were prepared and standard tests were applied to the porcelain tiles after single fast firing under industrial conditions. The densification behaviour and phase evolution of the bodies upon firing were evaluated. Physical, thermal and optical properties and chromatic coordinates were also measured. The results revealed that it was possible to employ the underclays to replace more expensive clay in porcelain tile production without deterioration of the required properties.