The methanol to DME reaction is promoted by the unique porous structure of zeolites, this provides a confinement effect that leads to transition state stabilisation. This work focuses on the MFI framework type structure (commonly known as ZSM-5), shown in fig. . This framework has orthorhombic symmetry (Pnma) within a large range of operating temperatures, and is composed of straight parallel pores (fig. ) intersected by sinusoidal pores (fig. ). These ordered pores provide large inter-nal surface areas which leads to the transition state stabilising confinement effect. Addition of organic carbonyl compounds further promotes the methanol to DME reaction allowing for use of milder reaction conditions; this reduces the risk of catalyst deactivation and side reactions in the hydrocarbon pool mechanism. The aliphatic esters and aromatic aldehydes investigated here have shown significant promotional effects (increasing catalytic activity by well over an order of magnitude) down to concentrations as low as 1 ppm in methanol. Our previous work employed a set of multilevel quantum calculations to investigate the action of aliphatic ester promoters in DME formation from methanol dehydration within H-ZSM-5. Our results showed how the promoters provide a more favourable route to DME formation. The promoters' adsorption to the reaction site is one crucial factor. Previously, we did not consider the role of diffusion; in the lead-up to the reaction, molecules must navigate the zeolite pores to access reaction sites and the reaction site becomes accessible for further reactions after product molecules diffuse away. At elevated temperatures the reaction potentially becomes diffusion limited, if the rate-limiting step shifts to diffusion of reactants in and products out of the reaction site. Many factors affect the diffusion of molecules in microporous solids, and molecular dynamics (MD) simulations have long been used to elucidate the dynamics of organic molecules in porous systems. Key to this is the flexibility of both the organic and inorganic components. 11 It is not possible to know a priori how zeolite flexibility will affect diffusion, so results with a fixed framework cannot be extrapolated to one which is flexible. Compounding this, recent work by Xu et al. has demonstrated flaws in the way some MD codes calculate temperatures and apply degrees of freedom for systems with rigid or frozen particles. For these reasons, in this paper we opt for simulations of flexible molecules within a flexible zeolite. Molecules can follow different diffusion pathways in zeolites depending on their size and shape. In the present work, we examine the diffusion through the different pores within ZSM-5 of methanol and the larger organic promoter molecules shown in Scheme 1. We investigate the spatial distribution of the promoter molecules. Diffusion coefficients are estimated and how these change when promoter and methanol are present together are examined. Novel behaviour, a molecular '3-point turn', is observed which we suggest enhances the manoeuvrability of the promoters and their navigation through the three dimensional zeolite network, allowing greater access to reaction sites.