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 depr...