The influence of inorganic material on the thermal deactivation of a biomass fuel and a coal was investigated. Reactivity experiments were performed for chars from wheat and leached wheat straw pyrolyzed at a wide range of heat treatment temperatures (HTT) in a thermogravimetric analyzer (TGA) and entrained flow reactor (EFR). The HTT were varied from 973 to 1673 K in the TGA, and 973 to 1613 K in the EFR. Reactivity experiments were also carried out for chars from the low rank coal, Dietz and demineralized Dietz, prepared in the TGA at the same conditions as the biomass chars. From the TGA data, the biomass chars show at any HTT a higher reactivity than the coal chars, but owing to thermal deactivation the reactivity of all fuel chars converge at the highest HTT. The influence of catalysts for char oxidation in both the biomass and the coal is significant up to HTT = 1273 K. Likely mechanisms for the loss of catalytic activity at high HTT in the TGA are discussed. The straw chars prepared in the EFR are on average a factor 30 to 40 times more reactive than the leached straw chars prepared at the same conditions in this reactor. SEM analysis of the EFR chars reveal that the main active catalyst, potassium, is found both in the organic matrix and in Si-rich regions of the char. Straw and leached straw produce chars that may have undergone a plastic stage during pyrolysis, similar to what is observed in coal chars. Using a common activation energy, the reactivity of the leached straw char prepared in the EFR at 1613 K is slightly higher than that of a typical char from a coal used in utility boilers (Cerrejon), suggesting that leached straw can be used in co-firing applications.