The influence of testing speed and temperature on the brittle-to-tough transition of non-adhering core-shell rubber-modified polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE) blends was studied. The validity of the concept of a network density dependent, critical matrix ligament thickness (IDc, as introduced in this series and verified mainly by slow-speed uniaxial tensile testing) is demonstrated for notched high-speed (1 m s-1) tensile testing at different temperatures. The influence of testing speed and temperature on the absolute value of IDc can be quantitatively understood in terms of a strain rate and temperature dependence of the yield stress. The simple model introduced in part 2 of this series proves to be valid under all testing conditions studied varying from temperatures of 50 to 150°C below the glass transition temperature of the PS-PPE blends. The absolute value of the tensile toughness, on the contrary, is a not yet quantified function of the test geometry applied and, consequently, cannot be directly derived from a simple strain-to-break argument.