Viscoelastic and forced Rayleigh scattering measurements were made on mixtures of PS and PVME over the wide range of temperature and PS molecular weight. The temperature dependence of the tracer diffusion coefficientDof PS in PVME was well described by the shift factoraTobtained from time‐temperature superposition of the dynamic shear modulus data of PVME in bulk. The dependence ofDon PS molecular weight was described by the free‐draining Rouse chain model in the lowMregion where the coil size of the diffusing PS chain is smaller than the mesh size of the PVME entanglement network. In the highMregion, on the other hand,Ddid not decrease in proportion toM−2, being in disagreement with the prediction of the reptation model, but took a constant value nearly equal to the calculatedDsvalue of the PVME molecule in bulk at the highMend. Two conjectures were proposed for interpretation of the above anomalous diffusion behavior.