Time dependence and the effect of polystyrene (PS) and poly(vinyl methyl ether) (PVME) molecular weight and polydispersity on interdiffusion at the interface of PS and PVME were investigated with attenuated total reflection infrared spectroscopy (ATR-FTIR). The time dependence of interdiffusion was studied by varying the thickness of the slow-diffusing component, PS, and the results were analysed using a combination of Fickian and Case II models. PS samples with molecular weights ranging from 1.0 x 10(5) to 3.0 x 10(6), above the entanglement molecular weight, were used to study the effect of molecular weight on interdiffusion. PS samples with controlled polydispersity ranging from 1.1-3.0 were prepared using a trimodal distribution constructed from monodisperse PS samples. The polydispersity of the PS samples was controlled by varying the number average molecular weight of the distributions while keeping the weight average molecular weight constant. To study the effect of PVME molecular weight and molecular weight distribution on interdiffusion, PVME was fractionated three times from an aqueous solution to increase its molecular weight from 9.9 x 10(4) to 1.7 x 10(5) and reduce its polydispersity index from 2.1 to 1.4, respectively.