ZSM-5 zeolites are used as additives in Fluidised Catalytic Cracking (FCC) units to increase the yield of LPG alkenes by conversion of gasoline-range hydrocarbons, forming a key link in the supply chain of these important feedstocks. Improving their yield is therefore a favoured route for meeting the current shortage of propene in the global petrochemicals market. Steam dealumination of the zeolite framework is known to result in changes in catalyst porosity over its operating lifetime in industrial use; these changes are believed to result in significant differences in the diffusion behaviour of hydrocarbon substrates between fresh and steamed zeolite. This proposal aims to compare the diffusion of propene and a model gasoline-range alkene (1-octene) in a simulated used catalyst to the behaviour previously observed in fresh zeolite (RB1810123).