The applicability of secondary fuels in practical plants (combustion, pyrolysis, gasification) requires a detailed characterization, mainly in severe thermal conditions, to provide optimized parameters for design and modeling purposes. Devolatilization is the basic step in all thermal treatments of materials. An experimental procedure is developed, described, and applied to the devolatilization of biomass (lignin-cellulosic materials), wastes (paper and sewage sludges), and low-quality coals. It consists of a preliminary characterization followed by further investigation (TG-FTIR analysis, kinetic abstraction, analysis on solid residue) to shed light on the effects of different operating conditions. The volatile matter released is found to strongly depend on the conditions used during the thermal treatment. Kinetics of devolatilization are obtained as functions of the heating rate in a wide range of conditions using different facilities on a laboratory scale (TG balance, wire mesh reactor, electrodynamic balance). This methodological approach provides important information and valuable parameters for practical applications and comprehensive modeling.