The deactivation of a standard 4-component FCC catalyst has been examined using a MAT reactor for a number of additives to a n-hexadecane feedstock. The additives included, benzofuran, indene, phenanthrene, quinoline and thianaphthene and were present at levels of 1% and 10% in the feedstock, Conversions decreased with all additives, but the most severe reduction occurred with quinoline, Benzofuran and indene gave most coke formation and analysis of the coke deposited on the catalyst indicated that this was initially mainly aliphatic in nature. A simple model has been developed for the coke deactivation process, which suggests that for the present results the major deactivation effect occurs in the production of C-5-C-6 hydrocarbons.