During biomass gasification, both the char physical properties and the biomass inorganic composition are known to have a significant influence on the reaction kinetics. However, the impact of the inorganic content is more pronounced than the impact of the char features, though no clear explanations have been found in the literature yet. In order to clarify this point, two biomass species with significant inorganic composition differences were gasified under steam and the chars obtained at various conversion values were characterized. Both the char physicochemical properties (chemical composition, carbon structure, porosity, and surface chemistry) and the gasification kinetic behavior were analyzed. A particular focus on the inorganic elemental composition and inorganic compound nature were performed. Experimental results were compared with simulation results at thermodynamic equilibrium obtained with the FactSage 7.2 software. The results showed that the physical properties of the carbon matrix do not have a strong influence on the gasification reactivity. In contrast, the inorganic composition could explain the differences between the gasification kinetic behaviors of the two investigated chars (obtained starting from two different biomass sources).