Thermal characteristics and model free kinetics of four different oil shale samples were studied using simultaneous thermogravimetry-mass spectrometer (TGA-MS) analysis performed at three different heating rates and under air atmosphere. All the reaction regions and corresponding peak temperatures, mass loss, and the residue of oil shale samples were determined. Meanwhile, the main volatile products, primary alcohols, and aromatic compounds as products of oil shale combustion, were determined on the basis of their relative intensities in different temperature intervals. Model free kinetics, particularly the Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn-Wall (OFW) methods, were used in order to determine the activation energy of the oil shale samples studied. It was observed that the activation energy of oil shale samples varied between 91.8 and 154.1 kJ mol(-1) depending on the type and origin of oil shale and model applied. The kinetic model is validated by comparing the experimental and simulated conversion curves.