A set of carbons has been made by pyrolyzing cellulose that has been oxidized in air for different amounts of time. The porosity of the carbons was characterized by N2 adsorption at 77 K, small-angle X-ray scattering (SAXS), and temperature-programmed desorption (TPD) of surface complexes formed during steady-state gasification in 5% NO gas. O2 and NO reactivities were determined for the carbons. It was found that the reactivity in O2 correlated well with N2 surface area, suggesting that all of the surface was active. However, no correlation was found between NO reactivity and BET surface area, which suggests that not all of the surface was active. NO reactivity correlated well with both the ratio of CO2:CO produced during the TPD experiments and surface fractal dimensions from the SAXS. The CO2 from the TPD experiments was due to a surface interaction between desorbed CO and nondesorbed C−O complexes. The TPD CO:CO2 ratio correlated well with fractal dimension, and high fractal dimensions were associated with the production of CO2. Therefore, fractal dimensions are a good indicator of the mass transfer properties of the pore systems. The fact that NO reactivity correlated well with surface fractal dimension suggests that mass transfer effects determine reactivity. The carbon with the highest reactivity was a mass fractal, which is associated with an open pore structure.