In the ferroelastic material WO3 it has been observed that diffusion of both sodium ions and oxygen vacancies occurs at an enhanced rate within twin domain walls. A number of explanations can be proposed for this result. Here we investigate how the structure of a ferroelastic domain wall can influence diffusion rates. We investigate a pair potential model of a ferroelastic system and determine the structure of the domain wall. We find that one parameter of the potentials gives rise to a coupling of the order parameter (in our model a shear strain) to a dilatational strain perpendicular to the domain wall. We calculate solution energies and hopping energies for isolated interstitial atoms in the limit of weak interaction with the lattice. Given the hopping energies we then numerically calculate concentration profiles resulting from diffusion from plane sources parallel and perpendicular to the domain wall and from a line source within the domain wall.