The competitive diffusion of components of the zeolite catalysed methanol to dimethyl ether (DME) process will be investigated using quasi-elastic neutron scattering. The diffusion coefficient of DME will be measured in a commercial sample of zeolite HY at temperatures between 300-400 K in the presence of D2O (the by-product) and CD3OD, methanol will also be measured in the presence of deuterated DME and D2O to understand the difference in diffusivity of the reactant and product mid-reaction. Measurements will also be taken of DME with D2O to investigate the diffusion behaviour upon reaction completion, and the diffusion of DME will be measured without D2O to understand the effect of by-product absence on diffusion. Results will be combined with measurements of pure methanol in the same zeolite sample completing the first neutron study of competitive diffusion in a catalytic process.