The effect of ultra-fine precipitated calcium carbonate (PCC), with a primary particle size of 50 nm, on the fusion behavior and mechanical properties of Ca/Zn stabilized poly(vinyl chloride) PVC has been investigated. The optimum concentration of PCC was determined and, in addition, experimental design was used to examine possible synergies between PCC and two important additives in an unplasticized PVC (PVC-U) formulation: namely, the processing aid and impact modifier. Dry blends were extruded and tested by differential scanning calorimetry, tensile testing, and Charpy impact testing. The results confirm that ultra-fine PCC can give rise to large improvements in single notch impact strengths. Control values of 8 kJ/m-2 were increased to 79 kJ/m-2 through the addition of ultra-fine PCC. There was also found to be a significant increase in fusion levels but no effect on the tensile strength of the samples. It is concluded that PCC behaves as both a processing aid and impact modifier in PVC-U formulations, and the reasons for this are discussed. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers