Fundamental information can be obtained by comparing the behavior of different classes of solid fuels (low- and high-volatile matter coals, biomass fuels, residues, wastes, and plastics) in oxidizing and inert thermogravimetric runs. Different mechanisms can be recognized and attributed to devolatilization, char oxidation, and direct oxidation. The direct oxidation of the fuel can be favored with respect to the traditional devolatilization/ char oxidation scheme depending on the conditions used and the characteristics of the fuel. Oxygen enhances the thermal devolatilization of the fuel and the reactivity of the produced char. The reactivity of all fuels are quantified by defining characteristic temperatures of each mechanism and providing kinetics for all steps. Also, the effect of the operating conditions are evaluated. Results obtained in this work represent a significant set of data on very different solid fuels, useful for modeling purposes, for practical system handling ( grate furnaces or fixed-bed combustors, operating in conditions similar to this study), for safety of fuel stockpiles, and for investigations on ignition and char reactivity.