The invention concerns a colloidal dispersion characterized in that it comprises an organic phase; particles of an iron compound in amorphous form and at least an amphiphilic agent. Said dispersion is prepared by a method which consists in reacting with a base either an iron salt in the presence of a complexing agent or an iron complex, while maintaining the pH of the reaction medium at a value of more than 8 whereby a precipitate is obtained, the iron complexing agent being selected among water-soluble carboxylic acids having a complexing constant K such that the pK is at least 3 and the iron complex being selected among reaction products of iron salts with said acids; then contacting the resulting precipitate or a suspension containing said precipitate with an organic phase in the presence of an amphiphilic agent whereby is obtained the dispersion in organic phase. The inventive dispersion can be used as combustion additive in fuels or liquid fuels.