The invention concerns a method for supplying iron, via the fuel, to the particulate trap of a diesel engine exhaust in a form suitable for promoting trap regeneration. The method involves the addition to the fuel of a defined colloid of iron oxide. Combustion of this colloid produces iron-containing compounds, especially iron oxides, which collect in association with carbonaceous particulate matter in the particulate trap, and function to promote the combustion of this matter.The colloid in particular shows a lower level of associated deposit formation on the fuel injectors than the iron additives of the prior art. The method is thus particularly suitable for modern engines showing increased susceptibility to fuel injector deposits. The colloid also shows a balance of properties providing excellent suitability for use as an additive in fuels, and especially in diesel engine on-board dosing devices.