The relationship existing between heat treatment of carbons, changes in their microstructural properties and loss of gasification reactivity is addressed. Experiments have been carried out on carbons of different ash content and composition and of different petrographic constitution. Samples have been heat treated under inert conditions at different temperatures (900-1400 degrees C) and for different times (1-300 min). Thermally induced changes of their microstructural properties have been characterized by the combined use of XRD, porosimetric analysis and SEM. Reactivity of the residual char after heat treatment has been measured by isothermal thermogavimetric analysis in carbon dioxide atmosphere at temperatures of 900 and 1200 degrees C. The analysis of the changes of gasification reactivity and of the microstructural properties upon annealing is directed to achieve a better understanding of processes that dominate char thermodeactivation, with particular emphasis on the relative importance of the organic versus inorganic matter transformations.