The percent conversion of sulfur, nitrogen, vanadium, nickel, aromatics, and CCR (Conradson carbon residue) has been analyzed as a function of residuum conversion for five series of reactions carried out in a batch reactor under nitrogen, hydrogen, or hydrogen in the presence of a residuum hydrotreating catalyst, over residuum conversions ranging from 38 to 86%. Sulfur, nitrogen, and CCR conversions could be divided into three categories, up to 50% residuum conversion, 50-70% residuum conversion, and greater than 70% residuum conversion. Sulfur conversion in the first category was from the distillable liquids and resulted from a combination of thermal, hydrogenation, and hydrogen atom addition reactions. Nitrogen removal in the first category was due to incorporation of nitrogen into reactor solids in the early stages of the reaction and was therefore suppressed by hydrogen in the absence of a catalyst. In the presence of hydrogen, CCR conversion was largely due to hydrogen atom addition. Without hydrogen, CCR conversion was negative through the first two categories of conversion. Aromatics in the residua fractions increased steadily as residuum Conversion increased, but increased faster in the coking case. Sulfur, nitrogen, and CCR conversions were all rapid after approximately 70% residuum conversion and were explained in terms of thermal reactions. Metals removal appeared to be due to thermal reactions with the solids in the reactor acting as a collector. Catalytic reactions did not appear to play a role. Overall, the main role of catalyst was the removal of sulfur from distillable liquids produced in the early stages of the residuum conversion reaction, and in CCR conversion, again in the early stages of the reaction.