The reaction rates of different sized of commercial cokes with CO₂ and air were measured using a combined thermogravimetric and differential thermal analysis technique. The temperature range for air oxidation was varied from 873 to 1073 K, and for the reactions with CO₂ it varied from 1173 to 1573 K. The oxidation rates were evaluated on the basis of the percentage weight loss and per unit surface area. Changes in reaction rate were found to depend on changes in the BET surface area.