We report the characterisation of zeolite ZSM-5 catalysts used in the conversion of dimethylether to hydrocarbons. Inelastic neutron scattering spectroscopy, supported by solid state NMR, shows that the more rapid deactivation occurring with dimethylether compared with methanol is associated with the formation of less methylated aromatic coke species and attributed to the lower levels of water present during dimethylether conversion. The ability of inelastic neutron scattering to probe a working catalyst with no sample preparation is demonstrated.