The diagenetic history of the Upper Eocene volcaniclastic rocks of the Thrace Basin is described based on petrological analysis of samples from six boreholes. Diagenesis in the volcaniclastic rocks principally involves the progressive development of various types of cements in the following order: quartz overgrowth, zeolite authigenesis, chlorite and illite authigenesis. After the formation of the cementation phases, there is a dissolution phase creating secondary porosity via the dissolution of volcanic class and feldspars, accompanied by generation of analcime and a late mordenite cementing phase. The volcaniclastic rocks also have been affected by the following diagenetic processes: fracturing and calcite, quartz, and zeolite cementation.