Evolution of volatiles and structural changes in green petroleum coke during its pyrolysis under nitrogen was studied by thermogravimetry in an induction furnace. Four cokes were examined in a particle size of 417-833 micon between 450 and 1200°C at heating rates of approximately 40, 90 and 150 K min - 1 . The weight loss and hydrogen and methane concentrations were measured, as well as the apparent and real densities, apparent porosities and crystallite sizes of coke samples pyrolysed to various degrees. A model which assumes a limiting-reaction-rate mechanism is used to represent data.