Hydrophobic fine solids can reduce the yield of toluene-insoluble coke in thermal cracking of heavy oils at short reaction times. In this work, fine carbon solids were used as additives in the coking of Arab heavy-vacuum residue (AHVR) in 1-methylnaphthalene (1-MN) to study the mechanism of this interaction. Both mesophase carbon and spherical graphite particles significantly reduced the coke yield compared with the case of no solid addition. Coke deposited on the surfaces of the two hydrophobic additives. This nucleation of coke deposition on the solid reduced the agglomeration of the coke, giving a better dispersion in the liquid. The highly dispersed coke phase on fine solids was more accessible for reactions with hydrogen-donor compounds in the oil phase, which in turn inhibited the initial rate of coke formation.