Different microstructures were obtained as a result of the chemical character of the polymer phases in barium sulfate-filled blends of polypropylene (PP) and poly(styrene-co-acrylonitrile) (SAN). The microstructure was controlled with the aid of low molecular mass maleic anhydride-grafted polypropylene (PP-g-MAH), which seems to form an interlayer around the filler particles resulting in controlling of the polymer phase in which the filler is occluded. The variation of the properties as a function of the concentration of PP-g-MAH with different amounts of grafted maleic anhydride and the average filler size was found to be due to a change of the microstructure. A low molecular weight PP-g-MAH interlayer around the filler particles occluded in the PP phase, dilution of the filler when occluded in the PP matrix phase of low viscosity as compared with SAN and filler network formation were found to be of importance for the dynamic mechanical properties. A theoretical model, the interlayer model, was used to simulate the dynamic mechanical properties of the filled blends.