A thermogravimetric study of coking of various C6–C7 hydrocarbons on HY and REY zeolites was performed using a Cahn microbalance modified to allow in‐situ measurements of catalyst weight changes during reaction under flow conditions. Mass transfer effects during the reaction were found to play an important role in the coking process of the static catalyst bed. The results indicate that catalytic coke can form in the void space between zeolite particles in such a way as to deter molecular diffusion through the catalyst bed.