In this study, organic geochemical characteristics and depositional environment of the Tertiary-aged oil shale deposits in Northwest Anatolia have been examined. Oil shales in all the studied areas are typically characterized by high hydrogen index and low oxygen index values. Beypazari, Himmetoglu, Golpazart and Bahcecik oil shales contain only Type I kerogen. Seyitomer and Hatildag shales contain dominantly Type I and minor amounts of Type II kerogen. Gas chromatography of Beypazart and Seyitomer oil shales shows only C-17 and C-18 n-alkanes but no other n-alkanes. Gas chromatography of Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales shows a bimodal distribution dominated by low-number n-alkanes. Pristan/Phytan (Pr/Ph) ratios of the shales suggest an anoxic depositional environment for Beypazari, Seyitomer and Hatildag and a suboxic environment for Himmetoglu, Golpazari and Bahcecik oil shales. T-max values indicate that oil shales are immature. In addition to gas chromatograms, the biomarker parameters measured using m/z 217 and m/z 191 mass chromatograms indicate that the oil shales contain immature organic matter. Gammacerane, a typical biomarker for saline depositional environment, has been determined in all the oil shales. When other biomarker parameters are taken into account, it is apparent that all the shale samples have been deposited in a saline environment. In general, the data obtained by geochemical analyses indicate a lacustrine depositional environment for all of the shale samples in the studied areas.