A novel experimental procedure is presented to evaluate changes in particle diameter and density during combustion under reaction-controlled conditions. Three samples with a mean diameter of 90 mum are employed, i.e., petroleum coke, graphite and gasifier residue. We carried out both thermogravimetry and particle imaging techniques under the same thermal conditions for each sample. It is revealed that petroleum coke particles shrink with a constant density, while the particle diameter and density simultaneously decrease for the others. In addition, the kinetics of the reaction order of petroleum coke with oxygen was determined in the temperature range 700-900 K. Reaction order in O-2 of 1.0 and activation energy of 157 kJ/mol are obtained. Combustion kinetics of graphite and gasifier residue are also discussed briefly.