In this study, effect of demineralization on the combustion reactivity of 25 lignite samples, originating from different areas of Turkey, was investigated. Non-isothermal thermogravimetry, where the sample was heated in air where temperature increased at a linear rate of 40 K/min to 1273 K, has been used to investigate the combustion reactivities of the original and demineralized lignite samples. The activation energies calculated by two different methods and the burning profiles of the original and demineralized samples were compared and discussed. It was observed that the combustion reactivity of the samples was considerably influenced by the demineralization process applied.