Diagenetic formation in the Haymana sandstones principally involves smectite and chlorite authigenesis, zeolite formation, and late Fe-calcite cement. There was a dissolution phase creating secondary porosity via the dissolution of quartz, carbonate, and feldspars. The latter processes are thought to be related to generation of acidic pore waters. The diagenetic history of the Upper Cretaceous Haymana sandstones is described based on petrological analysis of outcrop samples in the Yakacik vicinity. Consequently, Upper Cretaceous Haymana sandstones show poor reservoir quality.