This document relates to a fuel oil composition comprising: (i) a solid hydrocarbonaceous and/or solid carbonaceous material, wherein the material is in particulate form, and wherein at least about 90% by volume (% v) of the particles are no greater than about 20 microns in diameter; and (ii) a liquid fuel oil; wherein the solid hydrocarbonaceous and/or solid carbonaceous material is present in an amount of at most about 30 by mass (% m) based on the total mass of the fuel oil composition. The invention further relates a process for the preparation of this fuel oil composition, a method of changing a grade of a liquid fuel oil, and a method for adjusting the flash point of a liquid fuel oil.