This paper presents a detailed study of the air reactivity of petroleum cokes measured at temperatures between 400 and 600 degrees C using a combination of characterization techniques and reactivity measurements. The microstructure of the coke was found to comprise an essentially inaccessible pore system at low temperatures of 77-273 K used in characterization, and it is more accessible to oxygen at higher temperatures of about 773 K used in oxidation. The correlation of reactivity data using the random pore model suggests that the true micropore area is significantly larger than that measured using physical gas adsorption methods. The difference in surface area can be attributed to the low kinetic energy of gas molecules at the lower temperatures of characterization; as a result, they are unable to overcome the pore mouth energy barrier. By examining the variation of coke structure with burnoff level, we find that most of the internal reaction occurs in pores in the narrow pore width range 1-2 nm. For pores greater than 2 nm, however, the surface areas remains essentially constant with burnoff level. The apparent activation energy of the coke-air reaction derived from the extracted rate constants falls in the range 145-160 kJ/mol.