Heavy fuel oils, such as Bunker C, are widely used as fuels, particularly in marine engines for transportation. Considerable effort is being applied to reduce the smoke and stack solid emissions. These emissions are largely caused by the asphaltene content of the heavy fuel oils. New information concerning the nature and thermal decomposition of asphaltenes will enable progress to be made in the understanding of the pollutant routes for smoke formation. The emitted smoke is made up of unburned fuel, soot particles and cenospheres. There is evidence that asphaltenes reduce ignition delay times, but do not influence the burning times of the fuel droplets. The reduction in ignition delay is attributed to the production of volatiles from the pyrolysis of asphaltenes. A model is given outlining the kinetics of cenosphere formation during the combustion of the fuel oil droplet. (C) 2012 Elsevier Ltd. All rights reserved.