Steam gasification chars from lignocellulosic agrowastes are highly microporous materials with specific surface areas between 500 and 1000 m2/g, comparable to that of activated carbons. Nevertheless, the analysis and comparison of gasification chars from different feedstocks revealed the important effect of the raw biomass inorganic composition on their physicochemical properties. In particular, it was found that the catalytic effect of alkali and alkaline earth metals (AAEM) on the steam gasification reactions resulted in higher char specific surface areas, with greater proportions of surface oxygen-containing functional groups. In contrast, this effect was not observed for Si and P rich samples. In accordance, the biomass inorganic ratio K/(Si + P) proved to be a valuable indicator of the steam gasification char reactivity, giving a new insight for the engineering of useful value-added materials from lignocellulosic agrowastes. The mechanisms presented in this work could be an important reference for real gasification applications working with different kind of residues when the valorization of the solid byproduct is targeted.