The work focused on the elucidation of several key parameters in toughening poly(vinyl chloride) (PVC) by the methyl methacrylate-butadiene-styrene (MBS) core-shell particles. Accordingly, blends containing various weight percent of the MBS particles were prepared and characterized by dilute solution viscometry, dynamic light scattering, dynamic mechanical thermal analysis, transmission electron microscopy, and temperature variable impact test. The results showed PVC/MBS solution miscibility in almost all compositions with their maximum thermodynamic affinities at about 17 and 67 wt % of MBS in tetrahydrofurane (THF). In addition, MBS weight percent increase in its blend with the PVC above 10 led to severe impact energy raise with eventual leveling at about 17 wt %. Furthermore, blend toughness and its components miscibility in solution increased in parallel up to 20 wt % of MBS particles. On the other hand, blend toughness declined with test temperature decrease toward impact modifier core T-g at about -30 degrees C even for the sample with 20 wt % of the MBS particles. Finally, the brittle-ductile transition of the blend containing 20 wt % of the MBS particles comparison with its matrix tan delta-temperature correlation implied 2500 J/m impact energy equivalence with 90 degrees C sample temperature rise in secondary relaxation activation. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2691-2696, 2009