The present study was carried out using the microcarbon residue (MCR) test to investigate the mechanism by which vanadium- and nickel-containing additives cause a dramatic reduction in mosaic size in coke from delayed coker feeds. Since a fine mosaic microtexture is one of the key characteristics of shot coke, these additives have the potential to steer the morphology of the coke produced in a delayed coker drum to free-flowing shot coke. Midcontinent U.S. (MCUS) vacuum resid was selected because of its low metals content and its tendency to produce exclusively sponge coke in delayed coking. This allows us to easily observe changes in its shot-coke-forming tendency by monitoring the reduction in the microscopic domain size of the MCR coke using polarized light optical microscopy. The concentrations and chemical states of vanadium porphyrin, acetylacetonate and naphthenate, and nickel porphyrin were quantified using X-ray photoelectron spectroscopy before and after coking. The surface concentration is depressed for vanadium and nickel porphyrins added from 1000 to 10 000 atomic parts per million carbon atoms to MCUS vacuum resids. This observation is consistent with the behavior of native porphyrins in resids. The surface concentration of native vanadium and nickel porphyrins is depressed relative to the bulk in both petroleum residua and asphaltenes to the same degree as in feeds with additives. Following the coking of MCUS with vanadium and nickel porphyrin additives in the MCR test, the surface concentration gets closer to the bulk average. No evidence was found for the decomposition of any of the added vanadium and nickel porphyrins following coking. For resids, the surface concentration of vanadium acetylacetonate was severely depressed relative to the bulk; however, the surface concentration of vanadium naphthenate was enhanced relative to the bulk. Both vanadium acetylacetonate and vanadium naphthenate transformed into V2O5 following coking, and the surface concentration of vanadium was comparable to the predicted bulk average. All of the above-mentioned vanadium and nickel additives produced a coke having a fine mosaic domain structure characteristic of shot coke. The observation of similar coke morphology for soluble vanadium additives with very different chemical end products in the coke argues that additives have similar effects on the major hydrocarbon decomposition pathways. The disparity of vanadium and nickel concentrations at the surface is a new observation that confirms that petroleum resid is inhomogeneous at the microscopic level and asphaltenes self-associate via a mechanism involving secondary bonding interactions.