(57) A residual fuel oil composition comprises a residual fuel oil, preferably at No. 6 fuel oil, having dissolved therein an effective total amount of at least one zirconium salt of (i) an alcohol, or (ii) a phenol, each having a C₂ to C₂₄ hydrocarbyl group, or (iii) of a hydrocarbyl sulfonic acid such that the salt has a mol. wt of 100 to 2500, or of any mixture thereof; said effective total amount of the salt(s) being sufficient to reduce the amount of particulate matter which will be formed when, in use, the residual fuel oil is combusted. Suitably 1 to 1000 ppm of zirconium salt(s), calculated as metallic zirconium, are present. Preferred zirconium salts are zirconium propoxide or dodecyl benzene sulfonate. The composition is suitable for combustion in industrial boilers and refinery furnaces, preferably being fed in air or steam-atomized form.