Mordenite type zeolitic alteration was identified in the Upper Cretaceous andesitic, dasitic rocks in Sile region, Istanbul. Silisilication (quartz and opal-CT) and low degree clay alteration (smectite and illite) were recognized in mordenite-rich samples, while high degree clay alteration was determined in the samples which do not contain any mordenite mineralization. These alteration products have been developed in glassy componenis and also fill cavities and fissures of volcanic rocks. Besides, carbonate, chlorite and kaolinite type alterations were also identified in holes and in phenocrystols. Mordenite crystals display thick tabular or prismatic morphology, 40x80 μm long. Hexagonal cross sections of the crystals have thicknesses changing in orders of 10-40 μm. The average chemical composition of the mordenite crystals can be expressed by the chemical formula of 'Ca4.5(Al13, Si36, O96). 11 H2O'. DTA curves of mordenites show endotherms at 110°C and 560°C. According to the TGA curves of mordenites the total amount of weight loss below 950°C is about 11%.