Abstract In the present study, thermally induced changes in microstructure and CO2-gasification rates of fuel particles from Rhenish lignite and torrefied poplar wood are experimentally investigated. A literature review reveals a distinct gap in the field of quantitative analysis of thermal deactivation influencing low-rank and biogenic fuels reacting under gasification conditions. Therefore, the reactivity of Rhenish lignite and torrefied poplar wood towards carbon dioxide is examined after the process of heat treatment. Experiments are conducted in a small scale fluidized bed reactor. Assessment of thermal deactivation is achieved by varying heat treatment temperature between 1023–1173 K and heat treatment time between 0–1800 s. Flue gases are analyzed using a Fourier-transform infrared spectrometer to deduce a reaction rate from the temporal evolution of the product gas. Experimental results are reproduced by applying an nth-order power law and a model with distributed activation energies to propose parameters for implementation in comprehensive gasification/combustion codes and for comparison of these two model types: Both models approximate the experimental results almost equally well. In parallel to that, heat treated particles are analyzed by high resolution transmission electron microscopy. Thereby, a progressive transformation of the carbon from an amorphous microstructure to turbostratic arrangements is revealed for both types of fuel. In case of Rhenish lignite, formations of wrinkled carbon layers were detected, indicating a faster progress of transformation for lignite compared to biomass. This finding is in accordance with the kinetic study conducted, where the determined deactivation rate of lignite exceeds the one of biomass. In this work it is successfully shown that both reactivity and microstructure are affected by heat treatment on equal time scales.