Using oscillation mode of rheology and theoretical calculation, we have observed for the first time the crossover of mutual diffusion coefficient, D-m, from high to low temperatures at the multiple layers interface of polymer films. A model which reflects a more realistic terminal state has been proposed to fairly fit the experimental data, by which the mutual diffusion coefficient D-m can be determined. It is substantially found that the diffusion keeps proceeding for the multilayer system at the temperature lower than the critical temperature due to the requirement of a period of time for binodal compositions to reach. Moreover, it is found that the apparent activation energy, E-d, derived from the Arrhenius relation of D-m versus 1/T, increases surprisingly when the welding temperature is below 150 degrees C, which relates closely to the effects of the phase behavior occurring in the two-phase region of the blend. (C) 2010 Elsevier Ltd. All rights reserved.