PURPOSE: To obtain a combustion aid capable of uniformly dispersing coal powder into a fuel oil and also of improving the burning efficiency of a soil-liquid mixed fuel, by using mountain skin (clay mineral) carrying a heteropoly acid or its salt, or oxides of these.

CONSTITUTION: In a combustion aid for a solid-liquid mixed fuel, containing mountain skin carrying a heteropolyacid or its salt, or oxides of these, a carbonaceous combustible, e.g., coke, coal, asphalt, activated carbon, cellulose, wood, plastics, etc., is used as a solid component, and its preferable form is a powder of a particle size enough not to block the port of a burner, particularly a fine powder of a particle size of about 100 mesh or less. As a liquid component mineral spirit, kerosine, fuel oil, machine oil, organic solvents, water, etc., are used. Said mountain skin is a generic name of clay minerals containing a large number of active hydroxyl groups on the surface layer.