Poly(vinyl chloride) (PVC) was modified by addn. of corn. polymer-encapsulated Mg(OH)(2), Me3CCH2CHMe2 and EtCH Me-2-contg. microspheres (up to 2.5%) before extrusion in a five screw extruder (rotational speed 075-1.85 s(-1)). The amt., diam. and surface distribution of pores were detd. in 5 measuring areas of the PVC sample (from surface to the core). The no. of micro pores and their surface portion increased while the micropore diam. decreased with increasing amt. of microspheres used. Both no., surface portion and diam. of pores increased from surface area to core of the PVC sample.