Coupling between flow and diffusion at symmetric polymer/polymer interfaces has been investigated. Polystyrene/polystyrene sandwich assemblies were subjected to large-amplitude oscillatory shear (LAOS) using a sliding-plate rheometer (SPR) and the stress was monitored as a function of time. The results were treated using a new model combining Wagner's model with the theory of Doi and Edwards. The model explicitly expresses the influence of the strain and stress amplitudes frequency and time on the self-diffusion process. The apparent self-diffusion coefficient was found to increase with welding time, in agreement with our previous results obtained using small-amplitude oscillatory shear measurements. However, it was found in the present case that the self-diffusion coefficient depends strongly on the strain and stress amplitudes and frequency, and its steady state value was found to be larger than that determined from small-amplitude oscillatory shear measurements. It appears that the large strain oscillatory shear field continuously increases the density of chain ends at the interface and thus increases the flux of mass transport.