This research mainly investigates the decomposition behavior and kinetics of Beydili and Ihsangazi oil shale deposits (Türkiye) by TG-DTG and MS techniques. In the initial stage of the research, geochemical and organic characterization (total organic carbon, rock-eval, fisher assay, x-ray fluorescence, etc.) of the oil shale samples were determined. TG-DTG-MS analyses of the oil shale samples were performed at three different heating rates in an air atmosphere. In all the oil shale samples studied, the TG-DTG curves indicated that the decomposition of oil shale samples followed two successive reaction stages in different temperature intervals, known as pyrolysis and mineral decomposition. In these two stages, reaction intervals, peak temperature, mass loss, derivative mass loss rate values, and gaseous constituent (MS analysis) of oil shale samples are all determined. At the same time, the different combustibility indices, such as the ignition index, combustion index, and reactivity of oil shale samples, are also determined. Also, for each reaction region, activation energy values were calculated using two different model-free methods known as Kissinger - Akahira – Sunose (KAS) and Ozawa - Flynn – Wall (OFW). In pyrolysis and mineral decomposition regions, the activation energy values varied between 73.9 and 186.4 kJ/mol and 103.3–297.1 kJ/mol.