Kinetics of thermal pyrolysis of pitches from two heavy oil upgrading processes have been studied using thermogravimetric analysis. Experiments were carried out at atmospheric pressure with heating rates of 25-150 degrees C min(-1) to a final temperature of 800 degrees C. At temperatures below 150 degrees C, there is little devolatilization of either pitch. At higher temperatures, the pyrolysis takes place in two stages. Heating rate had a minor effect on the weight loss at a given temperature. Single-stage, first-order reaction methods, including models which incorporate a Gaussian distribution of activation energies, were found to be inadequate to interpret the thermogravimetric analysis results. A two-stage model which reflects changes in the chemical constitution or structure as conversion proceeds was necessary to describe the dependence of devolatilization rates on remaining volatile content. The transition between these two stages is a sharp one, occurring at about 450 degrees C for both pitches. The magnitudes of the activation energies suggest that both stages are kinetically controlled.