The effect of polystyrene (PS) matrix relaxation on interdiffusion at a PS and poly(vinyl methyl ether) (PVME) interface was investigated with attenuated total internal reflectance infrared spectroscopy (ATR-FTIR) at 105 and 85 degrees C, corresponding to 5 degrees C above and 15 degrees C below the T-g of PS, respectively. Monodisperse PS samples with ($) over bar M(w) of 3.0 x 10(6) and 1.05 x 10(5) were used to simulate relaxation in the PS matrix corresponding to chain reptation and constrained-release mechanisms, respectively. A binary blend of monodisperse PS samples consisting of short chains with ($) over bar M(w) of 3.0 x 10(4) and long chains with ($) over bar M(w) of 3.0 x 10(6) was used to simulate relaxation in the PS matrix by tube dilation. In the binary PS blend, the short chains were deuterated in order to monitor the concentration of each component independently. Non-Fickian diffusion was observed above and below the T-g of the PS matrix. Above the T-g of PS, interdiffusion was enhanced as the matrix relaxation mechanism changed from chain reptation to tube dilation to constrained release, consistent with the predicted relaxation time of these matrices. Below the T-g of PS, interdiffusion was controlled by the rate of swelling of PS by PVME and interdiffusion was faster in the binary PS blend due to the reduced entanglement density between the long chains.