This work was undertaken to determine whether thermodynamic calculations can predict the amount and composition of ash produced by combustion of nickel hyperaccumulator (HA) plants in the context of agromining. To this end, three HA plants containing about 1 wt% of nickel were submitted to isothermal combustion at 550, 815, 900 °C and the solid residues were characterized by complementary techniques (TGA, XRD, ICP-OES, ion chromatography, SEM-EDX), while thermodynamic calculations were performed with the FactSage software using the dry biomass composition as input. We present an in-depth evaluation of inorganics behavior based on the combination and comparison of experimental data and calculations: effect of temperature on ash yield, nature of crystallized phases, ash elemental compo­sition and inorganics volatilization. Our work confirms that equilibrium calculation is a powerful method to evaluate the behavior of inorganic elements during thermal treatment of biomass, despite some lacks in the databases. For the nickel HA plants, an innovative result is that, after combustion, nickel is always in the form of NiO particles, that can mix with MgO to form (Ni,Mg)O solid solutions. These results have direct implications for under­standing the combustion process and for better design of the following leaching step aiming at Ni recovery.