The structure of barium sulphate-filled immiscible blends with a polypropylene (PP) matrix can be controlled with respect to the occlusion of the filler with the aid of maleic anhydride-grafted polypropylene (PP-g-MAH) through the formation of an interlayer around the filler particles. Here we analyze the interlayer and the mechanism of interlayer formation in a blend with a poly(methylmethacrylate) (PMMA) dispersed phase and compare the results with previous studies, which concerned PP blends with polystyrene (PS) and poly(styrene-co-acrylonitrile) (SAN) minority phases. The main analytical tools were scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and Fourier Transform Infrared Spectroscopy (FTIR). The filler is occluded in the PMMA polymer in the PP/PMMA/BaSO4 (60/20/20 vol.%) blend, but is occluded in the PP phase at the addition of a sufficient amount of PP-g-MAH. The reason for the formation of the latter structure is a PP-g-MAH layer surrounding the filler particles, and the most likely mechanism behind this phenomenon is judged to be specific weak interactions between carbonyl groups in the graft copolymer and Ba2+ ions at the filler surface.