Abstract: A method of making a fuel for a portable heater comprises heating a salt of iron with a carboxylic acid at a temperature from about 400° C. to about 700° C. under an inert or reducing atmosphere to produce a composite material comprising carbon and iron nanoparticles. The fuel may be mounted to a substrate, such as a mesh or foil, and incorporated into a self-heating packaging. Typically, the packaging comprises a wall defining a chamber that, in use, contains a substance to be heated, e.g. a food-stuff or liquid. A heating manifold in thermal communication with the chamber contains the fuel in the inert atmosphere to initially inhibit production of heat otherwise arising from substantial oxidation of substantially all the fuel in the manifold in an exothermic reaction. A vent in the manifold is initially sealed and air-tight, but selectively opening of the vent allows oxidation of the iron nanoparticles.