The mechanical properties of extruded films of isotactic polypropylene (iPP) and ethylene-propylene rubber toughened iPP (iPP/EPR) have been analyzed in terms of the specific essential and non-essential work of fracture using high speed tensile test apparatus modified for quasi-static testing in the range 0.0001 to 3 ms.1. Three iPP/EPR grades with modifier contents of 15, 21 and 30 vol% were investigated. As the deformation rate increased, relatively uniform necking of the whole ligament and extensive plastic deformation were progressively replaced by more localized plastic deformation in the blends, and by fully brittle fracture in the unmodified iPP. The non-essential work of fracture and the total fracture energy were highly sensitive to these changes in deformation mechanism. However, the essential work of fracture, although dependent on the test speed, was less correlated with the extent of global plasticity.