Catalytic fast pyrolysis (CFP) using HZSM-5 zeolite catalysts serves as a promising route for the production of renewable chemicals from lignocellulosic biomass. However, this reaction is limited by the formation of coke, which can exceed 40% of carbon atoms present in the raw biomass feedstock. The role of structural parameters on coking remains unclear with both internal micropore diffusion and external mass transfer limitations hypothesized to actively contribute to carbon deposition. Here, we decouple the role of these parameters by comparing conventional in situ pyrolysis tests using model compounds to experiments performed with the reactant preadsorbed onto the zeolite catalyst. Experimental results supported by calculation of the mass transfer Biot number point to micropore diffusion as the dominant cause of coke formation. Specifically, the presence of defects such as internal crystal grain boundaries and extraframework species in the zeolite's micropores actively contribute to this undesired side reaction. Conversely, external surface barriers were found to play a minimal role in the deposition of carbon.