The gasification of demineralized chars and an activated carbon is studied. Detailed pore structure characterization of the carbonaceous materials is performed and their reactivity during gasification with oxygen and carbon dioxide is measured. Reactivity data are analyzed and compared to theoretical predictions for the evolution of surface area with conversion obtained from discrete models. The applicability of random pore models is critically analyzed and the effects of ordered pore structure and of non-uniform surface activity on the transient reactivity are investigated.