Phenomena taking place at the polymer-filler interface have been discussed and the effect of modifying chalk surface with stearic acid, chlorinated paraffin containing 42% Cl and with a (1:1) mixture of these modifiers on the bulk density, mass flow during extrusion and breaking stress of the rigid PVC - chalk compounds determined. An advantageous and synergistic effect of a mixture of stearic acid with chlorinated paraffin has been observed, what permitted an increase of chalk content in its compounds with PVC without deterioration of useful properties. From among three methods of introducing the modifiers into compounds the best results gave mixing in a fluidized-bed mixer directly during dry-blend preparation from PVC and chalk.