The present work extends previous investigations, relating to production of useful chemicals (especially levoglucosenone) from pyrolysis of wood treated with iron sulfate, by investigating the possibility of utilizing the resulting high-yield chars as gasification substrates. Gasification rates were measured by thermogravimetry after pyrolysis to form the chars in situ. The results show that sorbed ferrous and ferric sulfates are superior to indigenous inorganics in cottonwood as catalysts in O2 gasification of low-HTT (heat treatment temperature) chars (e.g., HTT 400-degrees-C) but are less effective than these indigenous species in high-HTT chars (e.g., HTT 850-degrees-C). In the case of CO2 gasification of the high-HTT chars, the added iron sulfates are superior to the indigenous species. For both types of gasification, ferric sulfate appears to be superior to ferrous sulfate as a catalyst. Apparent activation energies are calculated and reported. On the basis of this and other work, we conclude that the high-yield, low-HTT chars can be effectively utilized as substrates for O2 gasification, and that high-HTT char has potential as a substrate for O2 gasification and especially for CO2 gasification.