The paper presents the results of a study of the potential of modified kaolin as a filler for PVC. Kaolin from Maria III deposits in Poland was used. Aminosilanes and mercaptosilanes were used to modify the surface. Basic physicochemical parameters, such as the absorption capacity of the plasticiser, the heat of immersion in hydrophobic and hydrophilic solvents, and the morphology of particles of the modified filler were characterised. The modified kaolins clearly improved the physico-mechanical properties of PVC, in particular, they increased the elongation of PVC, when used at 10% and 15% w/w levels in the composite. Some studies of the microstructure of the filled composites were also performed using small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) techniques.