The activity, selectivity, and coke formation of a fluid catalytic cracking catalyst were investigated by cracking hexadecane (C-16) and a commercial feed (FCC) over Grace-Davison Super D Magnum catalyst. A microactivity test (MAT) unit was used in combination with a multiple cold-trapping (MCT) apparatus to generate time-dependent data. The reaction temperature was 930 degrees F and the cycle time was 3 min, during which the catalyst cracking activity decreased exponentially for both feeds. The final value of grams carbon/grams catalyst decreased in the order FCC/0.886>FCC/0.443>C-16. Interestingly, roughly 50% of the carbon formed was deposited within the first 20 s, and after 50 s the coking rates decreased by an order of magnitude. The catalyst activity versus carbon-on-catalyst curve shows three types of behavior. Initially, the catalyst is at its highest activity for both cracking and coking reactions. In the middle region, C-16 cracking deactivates quicker because the strong active sites required for cracking a paraffin of low carbon number have been fouled. In contrast, the commercial FCC feed is reactive enough to be converted over sites with a wider range of acid strengths. Finally, the cracking and coking rates reach their lowest value, but are above thermal rates. For the commercial FCC feed, the gasoline/C-1-C-4 gas ratio was found to increase until the coke rate became constant. No significant changes in selectivity during the reaction cycle were observed for C-16 cracking. From these data, it is proposed that C-16 coke formation occurs in series with the cracking reaction, while the commercial feed makes coke in reactions occurring parallel to the cracking reaction.